Refactor gsf.py to make the code of creating decoders and creating li…

…nk prediction dataloders reusable. (#825)

*Issue #, if available:*
Related to #789 

*Description of changes:*
1. decouple the code of creating decoders from the code of creating
built-in models to make the code of creating decoders more reusable.
2. add `get_builtin_lp_train_dataloader_class` and
`get_builtin_lp_eval_dataloader_class` in gsf.py


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Co-authored-by: Xiang Song <xiangsx@amazon.com>

python/graphstorm/__init__.py

@@ -28,3 +28,5 @@
 from .gsf import create_builtin_lp_model
 from .gsf import create_builtin_edge_model
 from .gsf import create_builtin_node_model
+from .gsf import (get_builtin_lp_train_dataloader_class,
+                  get_builtin_lp_eval_dataloader_class)