Fix sparse addmv ref impl for non-contig tensors

[malfet]

Fix logic in test_block_addmm that tested op against itself rather than against dense implementation, by implementing ref_addvm function that converts tensor back to dense before multiplying it with vector.

Fix reference implementation by passing stride for vector and result. (Not sure wether it will be more perf efficient to iterate over strided tensor or request a dense copy as MKL implementation does)

Print more verbose error message if values differ.

Fixes #97629 , #97589 , #97563

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[malfet]

@pytorchbot merge

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