[Grammar][Fix] Pass in stop tokens to xgrammar TokenizerInfo (#642)

Prior to this PR, using models such as SmolLM, which has `<|endoftext|>`
as an unk token and `<|im_end|>` as a stop token, runs into issues with
XGrammar. This is because XGrammar has a builtin set of stop tokens,
which includes `<|endoftext|>` but not `<|im_end|>`. This results in, at
the end of a structured generation, `<|endoftext|>` is forced to be
generated (as it is the only stop token recognized), but since it is not
an actual stop token, the generation of the model does not stop.

This PR explicitly passes in the stop tokens (recognized from
`mlc-chat-config.json`) to `createTokenizerInfo()` so we do not use the
built-in set of stop tokens. In the case above, `<|im_end|>` will be the
only stop token used by XGrammar, fixing the issue.

src/llm_chat.ts

@@ -554,6 +554,7 @@ export class LLMChatPipeline {
               this.token_postproc_method,
               this.prepend_space_in_encode,
               this.fullVocabSize,
+              this.stopTokens,
             );
             this.grammarCompiler =
               await xgr.GrammarCompiler.createGrammarCompiler(