reduce memory footprint for average_checkpoints (#647)

Summary:
Pull Request resolved: https://github.com/pytorch/fairseq/pull/647

the current implementation of average_checkpoints requires loading all
the model parameters into memory and then do the averaging. To average large
models (e.g., transformer) over a large number of checkpoints (e.g., >50),
it may require over 100GB memory.

Loading all the parameters is not necessary, as we know the number of models in advance.

Reviewed By: skritika

Differential Revision: D15027513

fbshipit-source-id: 0afe37c9a031a9ab0f1e78844a37be49ec5f76f1