[dynamo] Add BACKEND_MATCH guard to detect and recompile when backend changes (#107337)
**Motivation:**
We try to make torch.cond use torch.compile automatically so that we could error out when there is side-effects in the branches and correctly handle the closures.
Before this PR, we have a warning if we don't turn on a config raise_on_backend_change (turning it on gives us an error) for the following code:
```python
def foo()
# Inside torch.cond, we'd like to do something like
torch.compile(foo, backend="eager", fullgraph=True)(...)
...
# Users may then call torch.compile somewhere else.
# Dynamo will use the cached code of foo for "eager" backend
# but we expect dynamo to recompile with "inductor" backend.
torch.compile(foo, backend="inductor")(...)
```
This PR adds a BACKEND_MATCH guard. Effectively, it implements a per-backend cache. In the above example, the cached code for "eager" won't work for "inductor" due to guard check failures and the second torch.compile will do a re-compilation. In the future, it might be useful to have something like a configuration guard that guards against dynamo configuration changes across different compiles (e.g. compile a function with fullgraph=False then compile it again with fullgraph=True).
**Implementation:**
1. We add a guarded_backend_cache and check the most_recent_backend against the backend associated with cached code. We also remove the raise_on_backend_change flag.
2. Then newly added context manager and guard adds more lines for debug log so we change the uppper limit from 50 to 55.
**Test Plan:**
Removed original tests that raise on different backend and add a new test to test whether the BACKEND_MATCH guard can guard against backend change.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/107337
Approved by: https://github.com/jansel
