Add get_ids to GradStore (Discussion #2377)

[spaghetti-source]

Discussion in #2377

This PR adds get_ids function to GradStore that returns the list of TensorId in the store. It can be used to implement optimizers that only update relevant variables stored in the store. The typical use case is the word2vec model, which has many embeddings for the words but updates only a few in each iteration.

[LaurentMazare]

Thanks!