Integrate nested callable and getcallable

[muellerzr]

Integrate Nested Callable from fastcore

What does this add?

This PR merges the recent nested callable addition to fastcore into fastai.

Why is it needed?

The fastai codebase has multiple counts of a duplicated pattern:

getattr(obj, name, noop)()

We can instead use getcallable and nested_callable to remove that noop and call it implicitly.

This PR also changes in_channels to return the right StopIteration exception and just modifies the message, rather than a raw Exception class.

What parts of the API does this impact?

User-facing:

Nothing they will see

Internal structure:

All instances following the above pattern have been replaced with nested_callable or getcallable

cc @jph00

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