ENH: Adding reshape notation to einsum string

Proposed new feature or change:

I have implemented in about 30 LOC the following functionality for einsum:

import numpy as np
ab = np.random.normal(size=(2,3))
abcdefgh = np.random.normal(size=(2,3,4,5,6,7,8,9))
contracted = np.einsum('ab,abcdefgh->(cd)e(fgh)', ab, abcdefgh)  # <--- look at return string

assert contracted.shape == (4*5, 6, 7*8*9)
assert np.allclose(contracted, np.einsum('ab,abcdefgh->cdefgh', ab, abcdefgh).reshape(contracted.shape))

Note the return part of the string (cd)e(fgh): the idea is to use parentheses to enclose indices that we would like to reshape together.

I don't have time to follow up on docstring and tests. You can find the updated einsum function here

Start with the proposed implementation in numpy/_core/einsumfunc.py around line 1420 and review how existing einsum output notation is handled. Use the issue’s shape and np.allclose example to define expected behavior, then add the missing tests and docstring coverage noted by the author. Done means the parenthesized reshape notation works without regressing existing einsum behavior.