fix: serialize ContinuousBatchingConfig properly instead of null (#47039)

[webtecnica]

What does this PR do?

Fixes #47039 — ContinuousBatchingConfig was serializing as null in GenerationConfig.to_json_string() because it lacked a to_dict() method and its nested CompileConfig fields weren't being rehydrated after JSON round-trip.

Changes

src/transformers/generation/configuration_utils.py

1. ContinuousBatchingConfig.to_dict() — New method that serializes the instance to a dict, properly calling to_dict() on nested CompileConfig fields (varlen_compile_config, decode_compile_config).

2. ContinuousBatchingConfig.__post_init__() — Added rehydration of nested CompileConfig from dicts (handles the deserialization path after JSON round-trip).

3. GenerationConfig.__init__() — Added rehydration of ContinuousBatchingConfig from dict when a plain dict is passed (e.g., from from_pretrained after JSON deserialization).

tests/generation/test_configuration_utils.py

• Added test_serialize_generation_continuous_batching_config — Verifies:
  • Round-trip save/load preserves ContinuousBatchingConfig type and values
  • Nested CompileConfig fields are properly rehydrated
  • JSON output does not contain continuous_batching_config: null
  • _compile_all_devices is not leaked through CompileConfig serialization

Who can review?

@Rocketknight1 @remi-or

[github-actions]

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CI recap

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Latest run: 29689479036:2
Result: success | Jobs: 15 | Tests: 171,597 | Failures: 0 | Duration: 13h 54m