start work on schedule cache (#13529)

* start work on schedule cache

* local unique

* schedule cache works

* schedule cache cleanup

* fix tests

* preserve metadata

* oops, fix cache

* put that there

* fix spec

* always miss

* why is that broken?

* src[0].op

* fix process replay

* delete abstractions2

* reenable the actual schedule cache

* metadata is best effort

* fix JIT in examples/gradaccum_mnist.py

* full jit

* fixed and test is real

test/test_tensor.py

@@ -829,6 +829,7 @@ class TestTensorMetadata(unittest.TestCase):
     self.assertEqual(len(si.metadata), 3)
     self.assertEqual(set(m.name for m in si.metadata), {"relu", "sigmoid", "__mul__"})
 
+  @unittest.skip("metadata is no longer promised to be exact with schedulecache")
   def test_complex_backward(self):
     x = Tensor.rand(3, requires_grad=True).realize()
     y = Tensor.rand(3, requires_grad=True).realize()