Add `chunk_size` parameter to QGT constructors
Created by: attila-i-szabo
This PR adds a chunk_size
parameter to all three QGT constructors. These allow the QGTs to use a different chunk size than the underlying vstate, which is useful for some architectures, where backpropagation costs more memory than the forward pass. No existing behaviour changes, if chunk_size
is not supplied, the default is still vstate.chunk_size