Initial support for `nonstrict_trace` (#146367)
Summary:
## Context
> **Note:** `mark_traceable` got renamed to `nonstrict_trace` after
> offline discussion. The reasons are (1) it aligns with `torch.export`'s
> `nonstrict` notion, and (2) it's more definitive in behavior suggestion.
1. [Overall Design](https://docs.google.com/document/d/1O-dR2ZQaJQVt_v67AVcDCw2yJLtqgkZFwoXK0buEWRg/edit?tab=t.0)
2. [Dynamo graph representation with `torch._higher_order_ops.flat_apply`](https://docs.google.com/document/d/1YHl5nPTJvYeCPE5TO9uA18DPWNgUYGE4gCn6bFvXcBM/edit?tab=t.0#heading=h.xtw3hhbro4gn)
## Summary
This patch adds a `torch._dynamo.nonstrict_trace` decorator, which
currently is an enhanced version of `torch._dynamo.allow_in_graph` (see
docstring for their differences). Specifically, this patch focuses on
the UI and functionality prototyping/plumbing.
The main enhancement is supporting more input types, and the
implementation challenge lies in reconstructing the input objects from
Dynamo `VariableTracker` (while accounting for buffered side-effects and
guards). This patch takes a middle-ground (simple implementation with a
bit of user labor), by
1. asking the user to provide pytree registration for non-proxy-able
input types,
2. letting Dynamo trace through `pytree_flatten` (which accounts for
buffered side-effects and guards automatically),
3. and passing in the TreeSpec as a graph attribute constant into
`torch._higher_order_ops.flat_apply` (which unflattens the inputs and
invokes the underlying function).
## Next Steps
In subsequent patches, we will try to support the following:
- annotating on class method
- reads to global tensors
- inputs that contains `pytree.register_constant`-ed instances.
- function as input
- more output types (e.g., any pytree-registered type)
- `torch.nn.Module` as inputs
X-link: https://github.com/pytorch/pytorch/pull/146367
Approved by: https://github.com/zou3519
ghstack dependencies: #146714
Reviewed By: wdvr
Differential Revision: D70270803
fbshipit-source-id: d37fb2d7f79e2f7cc9e200e0c796f7087fe093bf
