sampling: add Top-nσ sampler (#11223)

sampling: add Top-nσ sampler (#11223)

* initial sampling changes:

* completed top nsigma sampler implementation

* apply parameter to only llama-cli

* updated readme

* added tests and fixed nsigma impl

* cleaned up pr

* format

* format

* format

* removed commented tests

* cleanup pr and remove explicit floats

* added top-k sampler to improve performance

* changed sigma to float

* fixed string format to float

* Update src/llama-sampling.cpp

Co-authored-by: Georgi Gerganov <redacted>
* Update common/sampling.cpp

Co-authored-by: Georgi Gerganov <redacted>
* Update src/llama-sampling.cpp

Co-authored-by: Georgi Gerganov <redacted>
* Update src/llama-sampling.cpp

Co-authored-by: Georgi Gerganov <redacted>
* Update src/llama-sampling.cpp

Co-authored-by: Georgi Gerganov <redacted>
* Update src/llama-sampling.cpp

Co-authored-by: Georgi Gerganov <redacted>
* added llama_sampler_init

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Co-authored-by: Georgi Gerganov <redacted>