support dynamic getitem in dictionary for symbolic values

[jjsjann123]

The old logic assumes getitem in dict is always a constant.

With a script like this:

def foo(a, v):
    return a[v].relu()

jfoo = thunder.jit(foo, cache="symbolic values")
a = {2: torch.randn(2, 2, device="cuda"), 8: torch.randn(3, 3, device="cuda")}
out = jfoo(a, 2)

Running into exceptions here:

NotImplementedError: Exception occured unpacking object from ProvenanceRecord(
  i1 = PseudoInst.INPUT_ARGS()
  i2 = PseudoInst.BINARY_SUBSCR(i1, 0)
  i3 = PseudoInst.BINARY_SUBSCR(i2, PseudoInst.CONSTANT([IntegerProxy name=i0, value=2, static=CONSTRAINT.CONSTRAINABLE]))
)

Since we now have PseudoInst.CONSTANT([IntegerProxy name=i0, value=2, static=CONSTRAINT.CONSTRAINABLE]), which tries to unwrap an IntegerProxy as constant.

The updated logic would preserve the wrapped value if it's holding a number proxy. So the above trace becomes something like this:

ProvenanceRecord(
  i1 = PseudoInst.INPUT_ARGS()
  i2 = PseudoInst.BINARY_SUBSCR(i1, 0)
  i3 = PseudoInst.BINARY_SUBSCR(i1, 1)
  i4 = PseudoInst.BINARY_SUBSCR(i2, i3)
)