Add SparseDot op to the MHLO dialect
Add SparseDot op to the MHLO dialect

- MHLO "sparsity" attribute represents SparsityDescriptor proto;
- MHLO "sparse_dot" op represents HloDotInstruction with sparsity descriptor;
- Adds transforms MHLO<->HLO;
- The verifier calls StableHLO inferDotGeneralOp helper.

Note: only N:M structured sparsity is supported for sparse dots at this point.
