fix: add missing shards_idx, preprocess_shards to docs and validator (#2331)

src/axolotl/utils/config/models/input/v0_4_1/__init__.py

@@ -169,6 +169,7 @@ class SFTDataset(BaseModel):
     type: Optional[Union[str, UserDefinedPrompterType]] = None
     input_transform: Optional[str] = None
     shards: Optional[int] = None
+    shards_idx: Optional[int] = None
     preprocess_shards: Optional[int] = None
     conversation: Optional[str] = None
     # Do not make this too strict or it will break the validator to choose different dataset class