[rollout,sglang] fix: get_tool_call_parser_type for gpt-oss models in sglang rollout (#3661)

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The problem is in the `get_tool_call_parser_type` function in
sglang_rollout.py (lines 225-246). The function is checking if
`parser.bot_token.strip()` exists as a single token in the tokenizer's
vocabulary, but for the gpt-oss parser type, the bot_token is
`<|start|>assistant<|channel|>commentary`, which is a compound token
sequence rather than a single special token. For gpt-oss models,
```
parser.bot_token.strip() = <|start|>assistant<|channel|>commentary
This gets tokenized as [200006, 173781, 200005, 12606, 815] (5 tokens)
```
The check parser.bot_token.strip() in tokenizer_vocab returns False
because it's looking for this entire string as a single vocabulary entry
The current logic assumes that bot_token should be a single special
token that exists in the vocabulary, but for GPT-OSS models, it's
actually a sequence of tokens that need to be tokenized.

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---------

Co-authored-by: Hejian Sang <[email protected]>
Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: 3 people

verl/workers/rollout/sglang_rollout/sglang_rollout.py

@@ -226,6 +226,10 @@ def get_tool_call_parser_type(
     processing_class: PreTrainedTokenizer | PreTrainedTokenizerFast | ProcessorMixin,
 ) -> str:
     items = FunctionCallParser.ToolCallParserEnum.items()
+    if "gpt-oss" in getattr(processing_class, "name_or_path", "").lower():
+        logger.debug(f"gpt-oss model detected from name_or_path: {processing_class.name_or_path}")
+        logger.debug("Using 'gpt-oss' tool call parser.")
+        return "gpt-oss"
     for parser_type, parser_cls in items:
         parser = parser_cls()
         try: