deps: add sentencepiece to the pi extra (FAST action tokenizer)

PI052 and PI0_FAST both load ``physical-intelligence/fast`` as
their action tokenizer. That tokenizer's HF backend requires
``sentencepiece`` to instantiate (or ``tiktoken``); without it
``AutoProcessor.from_pretrained`` raises:

  ValueError: Couldn't instantiate the backend tokenizer from one of:
  (1) a tokenizers library serialization file,
  (2) a slow tokenizer instance to convert or
  (3) an equivalent slow tokenizer class to instantiate and convert.
  You need to have sentencepiece or tiktoken installed [...]

It wasn't listed in pyproject so fresh installs missed it.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

pyproject.toml

@@ -129,6 +129,7 @@ av-dep = ["av>=15.0.0,<16.0.0"]
 pygame-dep = ["pygame>=2.5.1,<2.7.0"]
 placo-dep = ["placo>=0.9.6,<0.9.17"]
 transformers-dep = ["transformers==5.3.0"] # TODO(Steven): https://github.com/huggingface/lerobot/pull/3249
+sentencepiece-dep = ["sentencepiece>=0.2.0,<0.3.0"] # FAST action tokenizer backend (pi052, pi0_fast)
 grpcio-dep = ["grpcio==1.73.1", "protobuf>=6.31.1,<6.32.0"]
 can-dep = ["python-can>=4.2.0,<5.0.0"]
 peft-dep = ["peft>=0.18.0,<1.0.0"]
@@ -179,7 +180,7 @@ wallx = [
     "torchdiffeq>=0.2.4,<0.3.0",
     "lerobot[qwen-vl-utils-dep]",
 ]
-pi = ["lerobot[transformers-dep]", "lerobot[scipy-dep]"]
+pi = ["lerobot[transformers-dep]", "lerobot[scipy-dep]", "lerobot[sentencepiece-dep]"]
 smolvla = ["lerobot[transformers-dep]", "num2words>=0.5.14,<0.6.0", "accelerate>=1.7.0,<2.0.0"]
 multi_task_dit = ["lerobot[transformers-dep]", "lerobot[diffusers-dep]"]
 groot = [