This PR fixes Issue #32637 where ChatPromptTemplate's save method was raising NotImplementedError instead of inheriting the working implementation from BasePromptTemplate.

Issue

The ChatPromptTemplate class was overriding the parent class's working save method with a stub that just raised NotImplementedError. This prevented users from saving ChatPromptTemplate instances to files, even though the parent class BasePromptTemplate has a fully functional save implementation.

Root Cause

In src/transformers/processing_utils.py line 1303, there was an override:

def save(self, file_path: Union[Path, str]) -> None:
    """Save prompt to file.

    Args:
        file_path: path to file.
    """
    raise NotImplementedError

Solution

The fix removes this override, allowing ChatPromptTemplate to inherit the working save method from BasePromptTemplate. This enables users to save ChatPromptTemplate instances to JSON/YAML files as expected.

Testing

Added a test case that verifies:

1. ChatPromptTemplate.save() no longer raises NotImplementedError
2. The saved file contains valid JSON with the expected structure
3. The _type field is correctly set to 'chat'

Impact

This is a backward-compatible bug fix that restores expected functionality for ChatPromptTemplate serialization. Users can now save and load ChatPromptTemplate instances as documented, which is important for persisting prompt configurations.

Fixes #32637