use old cumsum optimization for arange (#3813)

revert to old cumsum opt while phi simplification is disabled.

added a flops complexity test for this

test/test_arange.py

@@ -0,0 +1,14 @@
+import unittest
+from tinygrad import Tensor, GlobalCounters
+
+class TestArange(unittest.TestCase):
+  def _get_flops(self, N):
+    GlobalCounters.reset()
+    Tensor.arange(N).realize()
+    return GlobalCounters.global_ops
+
+  def test_complexity(self):
+    f1 = self._get_flops(256)
+    f2 = self._get_flops(2560)
+    print(f"{f1=}, {f2=}")
+    assert f2 / f1 < 15, f"bad complexity, flops {f2/f1:.1f}X while inputs 10X"

tinygrad/tensor.py

@@ -268,7 +268,7 @@ class Tensor:
     if stop is None: stop, start = start, 0
     assert all(isinstance(s, (int, float)) for s in (start, stop, step)), "symbolic arange not supported"
     dtype = kwargs.pop("dtype", dtypes.default_float if any(isinstance(x, float) for x in (start, stop, step)) else dtypes.default_int)
-    return (Tensor.full((math.ceil((stop-start)/step),), step, dtype=dtype, **kwargs)._cumsum() + (start - step)).cast(dtype)
+    return (Tensor.full((math.ceil((stop-start)/step),), step, dtype=dtype, **kwargs).cumsum() + (start - step)).cast(dtype)
 
   @staticmethod
   def eye(dim:int, **kwargs):