Support tensor-likes

[patrick-kidger]

That is, classes supporting the __torch_function__ protocol.

This shouldn't be too difficult -- most of the necessary work has already been done.

• There's some places where we have torch.Tensor hardcoded, for example in instance checks and some type annotations, that would need adjusting to accept tensor-likes.
• TensorTypeMixin would need exposing as a public part of the interface.
• The documentation needs updating to show how this is possible. Once the above changes are made it should just be:

from torchtyping import TensorTypeMixin

class TensorLike:
    ...

class TensorLikeType(TensorLike, TensorTypeMixin):
    base_cls = TensorLike