Add Initial Support For Fake Tensor Constant Tracking (#84387)
Adds support for constant tensor tracking within FakeTensors. Copy-pasta'ing from `proxy_tensor.py` why this is useful:
```
# In some circumstances, we will be tracing in a situation where a tensor
# is *statically* known to be a constant (currently, this only happens if
# you run torch.tensor; deterministic factory functions like torch.arange
# don't get this treatment). When the tensor in question is small, it's
# helpful to due constant propagation in case we call item() (in which
# case we can return the constant value that is known, rather than give
# an error.)
```
This PR only attempts to add support for the tracing scenarios where we run each operation linearly - aot autograd, torchdynamo. It does not yet handle how constant tensors should be handled as part of the persistent fx graph. Additionally, it does not yet attempt to de-duplicate or interact with ProxyMode's only constant tensor handling.
Edit: plan is to rely on functionalization for fx graph
Pull Request resolved: https://github.com/pytorch/pytorch/pull/84387
Approved by: https://github.com/ezyang
