retrievers consume the query_vec

[cmacdonald]

Returning query_vec is useful for PRF. Is this enough of a use case to make it the default?

[seanmacavaney]

I worry about duplicating the vectors in memory k times per query --- we'll need to be careful with this implementation to make sure this doesn't happen (especially if the default).

[cmacdonald]

aiui, its k pointers, rather than k tensors, but we can check this.