feat: Add streaming support for batched prompts

[XBastille]

Description

Adds streaming support for batched prompts, resolving issue #406.

Changes

• Removed restriction that blocked batched streaming in _sampler.py
• Fixed _stream_sample_loop to wait for ALL batch elements to complete (not just first)
• Updated _stream_decode_state to properly handle batch dimensions
• Net reduction of 3 lines of code

Testing

• Tested with Gemma3_4B on NVIDIA L40s GPU
• Single prompt streaming: Works (unchanged)
• Batched non-streaming: Works (unchanged)
• Batched streaming: Now works (new feature!)

Backward Compatibility

Fully backward compatible - no breaking changes to existing API.

Fixes #406

[XBastille]

Hi @skandermoalla I noticed the repo has been a bit quiet since the last release, so I wanted to gently bump this. Since this PR fixes a functional issue #406 and is fully tested, I'd love to get it into the queue whenever the team is back to reviewing external contributions. Please let me know if I need to resolve any conflicts that have come up in the meantime. Thanks!