Add injection point before StructureModule

[ajtritt]

To add SAXS information via MSA/pair representations while leveraging the StructureModule, we will need to inject our modified representations before the StructureModule. Here are the two locations where this gets called.

1. This is for non-Ensemble AF iterations

• alphafold/alphafold/model/modules.py

  Line 151 in 7c09316

  ret[name] = head(representations, batch, is_training)

2. Ensemble AF iterations

• alphafold/alphafold/model/modules.py

  Line 334 in 7c09316

  ret[name] = module(representations, batch, is_training)