feat(turn-detector): drop transformers dependency

[chenghao-mou]

drop the transformers dependency in favor of a light hub + tokenizers combo.

benchmarked with the script on 500 files:

Op (mean)	en old	en new	intl old	intl new
render	43 μs	11 μs (~3.9x)	56 μs	17 μs (~3.3x)
encode	194 μs	85 μs (~2.3x)	260 μs	129 μs (~2.0x)
cold-load	1592 ms	361 ms (~4.4x)	896 ms	435 ms (~2.1x)

• en download files still works
• multi download files still works
• en inference still works
• multi inference still works

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 4 additional findings.