Implement `einsum` equivalent

[ricardoV94]

Please describe the purpose of filing this issue

Probably quite some work, but would be nice to have an equivalent: https://numpy.org/doc/stable/reference/generated/numpy.einsum.html

If we want to support the C backend:
https://numpy.org/devdocs/reference/c-api/array.html#c.PyArray_EinsteinSum

@aseyboldt previously linked to: https://optimized-einsum.readthedocs.io/en/stable/backends.html

[twiecki]

Would it be? Certainly JAX and numba have einsum implementations.

[ricardoV94]

The Op itself would be challenging to write, not so much the dispatch to other frameworks

[twiecki]

Why? It would just take a string and pass that through?

[ricardoV94]

You need to specify the input output types, and gradients and a few more stuff to make it not worse than the current manual alternatives.