openpilot1/tinygrad_repo/docs/quickstart.md

tinygrad Quick Start Guide

This guide assumes no prior knowledge of pytorch or any other deep learning framework, but does assume some basic knowledge of neural networks. It is intended to be a very quick overview of the high level API that tinygrad provides.

This guide is also structured as a tutorial which at the end of it you will have a working model that can classify handwritten digits.

We need some imports to get started:

import numpy as np
from tinygrad.helpers import Timing

Tensors

Tensors are the base data structure in tinygrad. They can be thought of as a multidimensional array of a specific data type. All high level operations in tinygrad operate on these tensors.

The tensor class can be imported like so:

from tinygrad.tensor import Tensor

Tensors can be created from an existing data structure like a python list or numpy ndarray:

t1 = Tensor([1, 2, 3, 4, 5])
na = np.array([1, 2, 3, 4, 5])
t2 = Tensor(na)

Tensors can also be created using one of the many factory methods:

full = Tensor.full(shape=(2, 3), fill_value=5) # create a tensor of shape (2, 3) filled with 5
zeros = Tensor.zeros(2, 3) # create a tensor of shape (2, 3) filled with 0
ones = Tensor.ones(2, 3) # create a tensor of shape (2, 3) filled with 1

full_like = Tensor.full_like(full, fill_value=2) # create a tensor of the same shape as `full` filled with 2
zeros_like = Tensor.zeros_like(full) # create a tensor of the same shape as `full` filled with 0
ones_like = Tensor.ones_like(full) # create a tensor of the same shape as `full` filled with 1

eye = Tensor.eye(3) # create a 3x3 identity matrix
arange = Tensor.arange(start=0, stop=10, step=1) # create a tensor of shape (10,) filled with values from 0 to 9

rand = Tensor.rand(2, 3) # create a tensor of shape (2, 3) filled with random values from a uniform distribution
randn = Tensor.randn(2, 3) # create a tensor of shape (2, 3) filled with random values from a normal distribution
uniform = Tensor.uniform(2, 3, low=0, high=10) # create a tensor of shape (2, 3) filled with random values from a uniform distribution between 0 and 10

There are even more of these factory methods, you can find them in the tensor.py file.

All the tensors creation methods can take a dtype argument to specify the data type of the tensor.

from tinygrad.helpers import dtypes

t3 = Tensor([1, 2, 3, 4, 5], dtype=dtypes.int32)

Tensors allow you to perform operations on them like so:

t4 = Tensor([1, 2, 3, 4, 5])
t5 = (t4 + 1) * 2
t6 = (t5 * t4).relu().log_softmax()

All of these operations are lazy and are only executed when you realize the tensor using .realize() or .numpy().

print(t6.numpy())
# [-56. -48. -36. -20.   0.]

There are a lot more operations that can be performed on tensors, you can find them in the tensor.py file. Additionally reading through abstractions.py will help you understand how operations on these tensors make their way down to your hardware.

Models

Neural networks in tinygrad are really just represented by the operations performed on tensors. These operations are commonly grouped into the __call__ method of a class which allows modularization and reuse of these groups of operations. These classes do not need to inherit from any base class, in fact if they don't need any trainable parameters they don't even need to be a class!

An example of this would be the nn.Linear class which represents a linear layer in a neural network.

# from tinygrad.nn import Linear
class Linear:
  def __init__(self, in_features, out_features, bias=True, initialization: str='kaiming_uniform'):
    self.weight = getattr(Tensor, initialization)(out_features, in_features)
    self.bias = Tensor.zeros(out_features) if bias else None

  def __call__(self, x):
    return x.linear(self.weight.transpose(), self.bias)

There are more neural network modules already implemented in nn, and you can also implement your own.

We will be implementing a simple neural network that can classify handwritten digits from the MNIST dataset. Our classifier will be a simple 2 layer neural network with a Leaky ReLU activation function. It will use a hidden layer size of 128 and an output layer size of 10 (one for each digit) with no bias on either Linear layer.

from tinygrad.nn import Linear

class TinyNet:
  def __init__(self):
    self.l1 = Linear(784, 128, bias=False)
    self.l2 = Linear(128, 10, bias=False)

  def __call__(self, x):
    x = self.l1(x)
    x = x.leakyrelu()
    x = self.l2(x)
    return x

net = TinyNet()

We can see that the forward pass of our neural network is just the sequence of operations performed on the input tensor x. We can also see that functional operations like leakyrelu are not defined as classes and instead are just methods we can just call. Finally, we just initialize an instance of our neural network, and we are ready to start training it.

Training

Now that we have our neural network defined we can start training it. Training neural networks in tinygrad is super simple. All we need to do is define our neural network, define our loss function, and then call .backward() on the loss function to compute the gradients. They can then be used to update the parameters of our neural network using one of the many optimizers in optim.py.

For our loss function we will be using sparse categorical cross entropy loss.

# from tinygrad.tensor import sparse_categorical_crossentropy
def sparse_categorical_crossentropy(self, Y, ignore_index=-1) -> Tensor:
    loss_mask = Y != ignore_index
    y_counter = Tensor.arange(self.shape[-1], dtype=dtypes.int32, requires_grad=False, device=self.device).unsqueeze(0).expand(Y.numel(), self.shape[-1])
    y = ((y_counter == Y.flatten().reshape(-1, 1)).where(-1.0, 0) * loss_mask.reshape(-1, 1)).reshape(*Y.shape, self.shape[-1])
    return self.log_softmax().mul(y).sum() / loss_mask.sum()

As we can see in this implementation of cross entropy loss, there are certain operations that tinygrad does not support. Namely, operations that are load/store or assigning a value to a tensor at a certain index. Load/store ops are not supported in tinygrad because they add complexity when trying to port to different backends and 90% of the models out there don't use/need them.

For our optimizer we will be using the traditional stochastic gradient descent optimizer with a learning rate of 3e-4.

from tinygrad.nn.optim import SGD

opt = SGD([net.l1.weight, net.l2.weight], lr=3e-4)

We can see that we are passing in the parameters of our neural network to the optimizer. This is due to the fact that the optimizer needs to know which parameters to update. There is a simpler way to do this just by using get_parameters(net) from tinygrad.nn.state which will return a list of all the parameters in the neural network. The parameters are just listed out explicitly here for clarity.

Now that we have our network, loss function, and optimizer defined all we are missing is the data to train on! There are a couple of dataset loaders in tinygrad located in /extra/datasets. We will be using the MNIST dataset loader.

from extra.datasets import fetch_mnist

Now we have everything we need to start training our neural network. We will be training for 1000 steps with a batch size of 64.

We use with Tensor.train() set the internal flag Tensor.training to True during training. Upon exit, the flag is restored to its previous value by the context manager.

X_train, Y_train, X_test, Y_test = fetch_mnist()

with Tensor.train():
  for step in range(1000):
    # random sample a batch
    samp = np.random.randint(0, X_train.shape[0], size=(64))
    batch = Tensor(X_train[samp], requires_grad=False)
    # get the corresponding labels
    labels = Tensor(Y_train[samp])

    # forward pass
    out = net(batch)

    # compute loss
    loss = sparse_categorical_crossentropy(out, labels)

    # zero gradients
    opt.zero_grad()

    # backward pass
    loss.backward()

    # update parameters
    opt.step()

    # calculate accuracy
    pred = out.argmax(axis=-1)
    acc = (pred == labels).mean()

    if step % 100 == 0:
      print(f"Step {step+1} | Loss: {loss.numpy()} | Accuracy: {acc.numpy()}")

Evaluation

Now that we have trained our neural network we can evaluate it on the test set. We will be using the same batch size of 64 and will be evaluating for 1000 of those batches.

with Timing("Time: "):
  avg_acc = 0
  for step in range(1000):
    # random sample a batch
    samp = np.random.randint(0, X_test.shape[0], size=(64))
    batch = Tensor(X_test[samp], requires_grad=False)
    # get the corresponding labels
    labels = Y_test[samp]

    # forward pass
    out = net(batch)

    # calculate accuracy
    pred = out.argmax(axis=-1).numpy()
    avg_acc += (pred == labels).mean()
  print(f"Test Accuracy: {avg_acc / 1000}")

And that's it

Highly recommend you check out the examples/ folder for more examples of using tinygrad. Reading the source code of tinygrad is also a great way to learn how it works. Specifically the tests in test/ are a great place to see how to use and the semantics of the different operations. There are also a bunch of models implemented in models/ that you can use as a reference.

Additionally, feel free to ask questions in the #learn-tinygrad channel on the discord. Don't ask to ask, just ask!

Extras

JIT

Additionally, it is possible to speed up the computation of certain neural networks by using the JIT. Currently, this does not support models with varying input sizes and non tinygrad operations.

To use the JIT we just need to add a function decorator to the forward pass of our neural network and ensure that the input and output are realized tensors. Or in this case we will create a wrapper function and decorate the wrapper function to speed up the evaluation of our neural network.

from tinygrad.jit import TinyJit

@TinyJit
def jit(x):
  return net(x).realize()

with Timing("Time: "):
  avg_acc = 0
  for step in range(1000):
    # random sample a batch
    samp = np.random.randint(0, X_test.shape[0], size=(64))
    batch = Tensor(X_test[samp], requires_grad=False)
    # get the corresponding labels
    labels = Y_test[samp]

    # forward pass with jit
    out = jit(batch)

    # calculate accuracy
    pred = out.argmax(axis=-1).numpy()
    avg_acc += (pred == labels).mean()
  print(f"Test Accuracy: {avg_acc / 1000}")

You will find that the evaluation time is much faster than before and that your accelerator utilization is much higher.

Saving and Loading Models

The standard weight format for tinygrad is safetensors. This means that you can load the weights of any model also using safetensors into tinygrad. There are functions in state.py to save and load models to and from this format.

from tinygrad.nn.state import safe_save, safe_load, get_state_dict, load_state_dict

# first we need the state dict of our model
state_dict = get_state_dict(net)

# then we can just save it to a file
safe_save(state_dict, "model.safetensors")

# and load it back in
state_dict = safe_load("model.safetensors")
load_state_dict(net, state_dict)

Many of the models in the models/ folder have a load_from_pretrained method that will download and load the weights for you. These usually are pytorch weights meaning that you would need pytorch installed to load them.

Environment Variables

There exist a bunch of environment variables that control the runtime behavior of tinygrad. Some of the commons ones are DEBUG and the different backend enablement variables.

You can find a full list and their descriptions in env_vars.md.

Visualizing the Computation Graph

It is possible to visualize the computation graph of a neural network using graphviz.

This is easily done by running a single pass (forward or backward!) of the neural network with the environment variable GRAPH set to 1. The graph will be saved to /tmp/net.svg by default.