Fix zero-dim handling in torch.matmul (#63359)

Fix zero-dim handling in torch.matmul (#63359)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/63359

Fixes #63352. The problem was that in e.g. `torch.matmul(A, B)` with A,
B having shapes [3, 2, 0] and [0, 2], the code attempts to call
`A.view(-1, 0)` which fails due to "-1 being ambiguous". The solution is
to manually compute what we want the shape of the view to be.

Test Plan: - new tests

Reviewed By: ngimel

Differential Revision: D30351583

Pulled By: zou3519

fbshipit-source-id: 7625691fe8b85d96a4073409596a932c303e3e8c