RFC: add Hermitian transpose function

[asmeurer]

See for instance https://github.com/numpy/numpy/issues/13797. It might be useful to have a special function for a hermitian transpose, which would be equivalent to conjugate(matrix_transpose(x)).

The other suggestion is a .H operator on matrices. A complication here is that libraries that use views would not be able to make .H a view unless they add a special conjugate-complex dtype. See the discussion at https://github.com/numpy/numpy/issues/13797. (the .H attribute is really the bigger thing here, as it least to more readable code, but if it is added it probably makes sense to also add a corresponding function)

[shoyer]

A complication here is that libraries that use views would not be able to make .H a view unless they add a special conjugate-complex dtype.

It's worth noting that libraries that use JIT compilers (e.g., JAX and PyTorch) will likely be able to optimize these operations even without explicit views.

[rgommers]

PyTorch has a special "complex conjugate bit", which makes torch.Tensor.H an O(1) operation even in eager mode. I believe the conclusion from the NumPy discussion was that something like that is also desired if .H is going to be a thing (implemented as special dtype - the details probably don't matter, just that it's a lot of work and it's not moving forward).

I'd prefer not to touch .H, because we can't really adopt it without giving NumPy (and perhaps other libraries) the choice between (a) doing a lot of work, (b) not being compliant, or (c) letting go of a pretty fundamental design rule that attribute access should be cheap.

It might be useful to have a special function for a hermitian transpose, which would be equivalent to conjugate(matrix_transpose(x)).

This seems easier to do.