[feat] Improve the weight conversion interface

[jlamypoirier]

🧐 Problem Description

There are minor issues with the weight conversion. Mainly:

• It's not consistent with ParamConverter, for example fast_llm_names with enforced tuple format vs fast_llm_name that accepts both tuple and str format.
• The external converter is hard-coded to Safetensors. We probably want to support other file formats.

See also #98

💡 Proposed Solution

• Make variable names plurial in WeightConverter.
• Enforce tuple format
• Generalize StateDictCheckpointHandler so it's not hard-coded to Safetensors

🔄 Alternatives Considered

Things work right now so we don't have to do anything, but it's a good idea to be proactive.

📈 Potential Benefits

Improved consistency and generalizability.