Add documentation for a catching invalid index type (#96451)
Summary:
The assert in in compute_q8gemm_prepacked_sparse_dq is currently unreachable.
Added inline comments to explain what is happening.
Test Plan: Ran qnnpack q8gemm-sparse-test to verify.
Differential Revision: D43930667
Pull Request resolved: https://github.com/pytorch/pytorch/pull/96451
Approved by: https://github.com/salilsdesai, https://github.com/jianyuh
