Dev release of v3

Note that future release and dev releases of v3 might be backwards incompatible with this dev release.

This dev release:

• custom_m renamed to transform_module
• transform_module always used now (the weight and weight_proj parameters have been removed)
• projection_size can take in a sequence of projection_sizes, one per layer
• n_proj in SRUCell renamed to projection_size, for consistency