Make parameters config optional in RawPyTorchMergeConfig. (#630)

In contrast to `merge-yaml` the `merge-pytorch` configuration has
non-nullable `parameters` value ([MergeConfiguration
L99](https://github.com/arcee-ai/mergekit/blob/main/mergekit/config.py#L99)
vs [RawPyTorchMergeConfig
L31](https://github.com/arcee-ai/mergekit/blob/main/mergekit/scripts/merge_raw_pytorch.py#L31)).

The change adds possibility to provide empty `parameters` value during
`merge-pytorch` and passes all the tests.

Author: mklimasz

mergekit/scripts/merge_raw_pytorch.py

@@ -28,8 +28,8 @@ class InputModelDefinition(BaseModel, frozen=True):
 
 class RawPyTorchMergeConfig(BaseModel, frozen=True):
     merge_method: str
-    parameters: Optional[Dict[str, ParameterSetting]]
     models: List[InputModelDefinition]
+    parameters: Optional[Dict[str, ParameterSetting]] = None
     dtype: Optional[str] = None
     base_model: Optional[str] = None
 
@@ -171,7 +171,7 @@ def construct_param_dicts(
 ):
     global_params = {}
     for param_def in merge_method.parameters():
-        if param_def.name in config.parameters:
+        if config.parameters and param_def.name in config.parameters:
             value = evaluate_setting(tensor_name, config.parameters[param_def.name])
             if value is not None:
                 global_params[param_def.name] = value
@@ -194,7 +194,8 @@ def construct_param_dicts(
             ):
                 tensor_params[mr][param_def.name] = value
             elif value := evaluate_setting(
-                tensor_name, config.parameters.get(param_def.name, [])
+                tensor_name,
+                config.parameters.get(param_def.name, []) if config.parameters else [],
             ):
                 tensor_params[mr][param_def.name] = value
             elif param_def.required: