[Cleanup] Delete unused methods / imports / args / classes: 2/N

Created by: suchenzang

Following https://github.com/facebookresearch/metaseq/pull/230 and continuing to break down https://github.com/facebookresearch/metaseq/pull/197.

Removed:

• unused moe_disable_padding arg
• unused from_pretrained methods, since we currently depend on load_model_ensemble_and_task from checkpoint_utils (not great but saving that for another PR)
• unused hub_models method
• unused return_all_hiddens arg, and no encoder_states being returned as a result
• EncoderDecoderModel class