Add input information to fusion definitions for trace inspection and debugging

[riccardofelluga]

Before submitting

• [ ] Was this discussed/approved via a Github issue? (no need for typos and docs improvements)
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• [ ] Did you make sure to update the docs?
• [ ] Did you write any new necessary tests?

What does this PR do?

Fixes #387. This PR adds information about the inputs for a fusion definition such that it can be retrieved by inspecting the trace. A tutorial on how to read this information will be published as part of #205.

Also this PR is in preparation for #205

Quickly, from a trace:

trace = thunder.last_traces(fn)[-1]
trace_ctx = trace.python_ctx()
print(trace_ctx['nvFusion0'].last_inputs())

will print something like:

inputs = [
    torch.randn((2048,), dtype=torch.float32, device='cuda:0').as_strided((1, 2048), (2048, 1)),
    torch.randn((4096,), dtype=torch.bfloat16, device='cuda:0').as_strided((1, 2048, 4096), (4096, 0, 1))
]

I'm open to change the str output to returning a list of tensors, however for debugging it's usually enough to have a string.