Add einsum for Qobj.

[Ericgig]

Problem Description

Having a version of einsum working on Qobj's dimensions would be useful (qutip/qutip-qip#225).
A dense implementation should be easy enough to develop using qutip.core.dimension.to_tensor_rep.

Proposed Solution

Use to_tensor_rep, from_tensor_rep with np.einsum to create qutip version of einsum.

Alternate Solutions

If would be even better to develop it without conversion to numpy array to allow support for sparse, jax, cupy, etc. matrices.
However this would probably not be an easy task doable in the scope of good first issue.

Additional Context

No response