Added intermediate dim change

Added intermediate ff dimension

Now, the model dimension can be different in the intermediate layers.
This change applies to the ff module, and only in the encoder. Now, if
the flag ff_intermediate is not None, the layers will look like this:

channels -> ff_dim -> ff_intermediate (For layer 1)
ff_intermediate -> ff_dim -> ff_intermediate (For layers 2 to depth-1)
ff_intermediate -> ff_dim -> channels (For layer depth)

As opposed to

channels -> ff_dim -> channels (For all layers)