Raise error in parse_einsum_input when output subscript is specified multiple times

[lgeiger]

Description

contract("ij->jij", [[0, 0], [0, 0]])
# ValueError: einstein sum subscripts string includes output subscript 'j' multiple times

currently relies on the backend to throw an error if an output subscript is specified multiple times. However

contract_path("ij->jij", [[0, 0], [0, 0]])

would still return an einsum path despite the wrong einsum equation. This might lead to subtle errors if a user ends up relying on this behaviour by accident. E.g. when using the jax backend no error is thrown but only an internal assertion fails.

This PR raises the error already in parse_einsum_input which ensures that contract_path matches the behaviour of np.einsum and all backends that might rely on opt_einsum for error handling will do so as well.

This PR also extends the unittests to test error handling for both contract_path and contract which is independent of the backend.

For reference I made a similar PR to numpy/numpy#25230 to fix the same issue for np.einsum_path.

Status

• Ready to go

[codecov]

Codecov Report

Merging #222 (2e51e2b) into master (1a984b7) will increase coverage by 3.15%.
The diff coverage is 100.00%.

Additional details and impacted files

[dgasmith]

Apologies for missing this, looks like a great change- especially in testing!