Add MRR (Mean Reciprocal Rank) metric to rec_sys

[steaphenai]

closes #3569
Refs #3572

Description: Add MRR (Mean Reciprocal Rank) metric to ignite/metrics/rec_sys.

• Supports multiple top_k values in a single call
• Uses torch.argsort with stable=True for deterministic tie-breaking
• Uses ranx as reference implementation for tests
• Adds relevance_threshold to support graded relevance labels

Check list:

• New tests are added (if a new feature is added)
• New doc strings: description and/or example code are in RST format
• Documentation is updated (if required)

[vfdev-5]

@steaphenai please check also this discussion on top k stuff: #3568 (comment)

[steaphenai]

@steaphenai please check also this discussion on top k stuff: #3568 (comment)

Thanks! I've read #3568. The current implementation keeps top_k specific to MRR following the same pattern as HitRate. I'll update if needed once the TopK wrapper approach is decided.