refactor: update AtomsDataModule deprecation warning and configuration path

Author: sundusaijaz

src/schnetpack/configs/data/custom.yaml

@@ -1,5 +1,5 @@
 # @package data
-_target_: schnetpack.data.datamodule_v2.AtomsDataModuleV2
+_target_: schnetpack.data.datamodule.AtomsDataModule
 
 dataset:
   _target_: schnetpack.data.ASEAtomsData

src/schnetpack/data/datamodule.py

@@ -78,7 +78,7 @@ def __init__(
 
         if used_legacy_args:
             warnings.warn(
-                "The following arguments are deprecated in `AtomsDataModuleV2`: "
+                "The following arguments are deprecated in `AtomsDataModule`: "
                 f"{used_legacy_args}. "
                 "Use a prebuilt dataset instance and configure these options on the "
                 "dataset instead.",