feat(pi0-fast): support automatic tokenizer fitting

tests/policies/pi0_fast/test_pi0_fast_tokenizer_fit.py

@@ -0,0 +1,63 @@
+#!/usr/bin/env python
+
+# Copyright 2026 The HuggingFace Inc. team. All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from lerobot.policies import factory
+from lerobot.policies.pi0_fast import processor_pi0_fast
+from lerobot.policies.pi0_fast.configuration_pi0_fast import PI0FastConfig
+from lerobot.policies.pi052 import fit_fast_tokenizer as fit_module
+
+
+def test_pi0_fast_resolves_dataset_specific_tokenizer(monkeypatch, tmp_path):
+    config = PI0FastConfig(
+        auto_fit_fast_tokenizer=True,
+        action_tokenizer_name="base-tokenizer",
+        fast_tokenizer_cache_dir=str(tmp_path),
+        fast_tokenizer_fit_samples=17,
+        chunk_size=12,
+        n_action_steps=12,
+    )
+    received = {}
+
+    def fake_fit(**kwargs):
+        received.update(kwargs)
+        return "/cache/fitted-tokenizer"
+
+    monkeypatch.setattr(fit_module, "fit_fast_tokenizer", fake_fit)
+
+    assert fit_module.resolve_fast_tokenizer(config, "user/dataset") == "/cache/fitted-tokenizer"
+    assert received == {
+        "dataset_repo_id": "user/dataset",
+        "cache_dir": tmp_path,
+        "base_tokenizer_name": "base-tokenizer",
+        "n_samples": 17,
+        "chunk_size": 12,
+    }
+
+
+def test_pretrained_pi0_fast_rebuilds_processor_only_during_dataset_fit(monkeypatch):
+    config = PI0FastConfig(auto_fit_fast_tokenizer=True)
+    expected = (object(), object())
+
+    monkeypatch.setattr(processor_pi0_fast, "make_pi0_fast_pre_post_processors", lambda **_: expected)
+
+    assert (
+        factory.make_pre_post_processors(
+            config,
+            pretrained_path="checkpoint",
+            dataset_repo_id="user/dataset",
+        )
+        == expected
+    )