Add specification for computing the matrix transpose (matrix_transpose)

[kgryte]

This PR:

• adds a specification for computing the transpose of a matrix (or a stack of matrices).

Background

The addition of a matrix_transpose API is meant to replace the use of the transpose API. The transpose API was found to be problematic due to its API supporting more general axes permutation (see gh-228).

Similar to other linear algebra APIs, the proposed matrix_transpose API operates on stack of matrices and thus more succinctly and explicitly allows expressing matrix operations in accordance with mathematical intuition and obviates the need for doing transpose(x, axes=[-2,-1]) whenever wanting to perform a batched matrix transpose. Batching is the default behavior.

The matrix_transpose API serves as a functional complement to the proposed .mT attribute (see gh-246).

[kgryte]

Adding the matrix_transpose was discussed in the consortium meeting held 09-02-2021. No objections were raised, as this PR provides the desired behavior of batched matrix transpose.

Merging...