Make proxy tensor support item() calls on torch.tensor constants (#81192)
This PR is doing a few interrelated things, all of which are necessary to get correctness. Read the comment in torch/fx/experimental/proxy_tensor.py for the high level overview.
Let's break down the parts of this PR:
* Bug fix where `enable_torch_dispatch_mode` with `None` doesn't work. This make `enable_torch_dispatch_mode(current_mode.inner)` work which is the basis for how we temporarily disable fake tensor mode.
* Bug fix for when fake tensor mode is combined with a non-mode tensor subclass. This actually could be ablated from this PR but it affects where the logic for allowing non fake tensor inputs with lift goes, so it's all in here in one go. There are some relevant tests for the fix in fake tensor, but it turns out I didn't need this because I'm always using proxy tensors as a mode (which ensures the ordering is right.)
* New `lift_fresh` view operator. Note that like lift, we have to manually write the functionalize kernel for these functions.
* The actual change, which is to save constants when we see them in the proxy tensor mode, and then propagate them as we go (because otherwise you'll handle mutations on constants incorrectly--see test.)
This is mildly BC-breaking if anyone was previously interposing on
at::lift, but this operator was relatively new and I checked
functorch which has no explicit reference to lift. So I think it
should not be too disruptive.
Signed-off-by: Edward Z. Yang <ezyang@fb.com>
Pull Request resolved: https://github.com/pytorch/pytorch/pull/81192
Approved by: https://github.com/samdow, https://github.com/bdhirsh
