Add additional shape validation to `compute_accidental_hits`

In `compute_accidental_hits`, the `sampled_candidates` must
be a vector, as is shown in the kernel implementation in
`tensorflow/core/kernels/candidate_sampler_ops.cc`.

This fix adds shape validation of `sampled_candidates`
in the shape function whenever possible.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>