Remove defaults for arch - force users to specify

[suchenzang]

There are lots of defaults set for all the transformer_* arches - we should remove as much of these as possible and noisily fail if users do not specify these values.

https://github.com/facebookresearch/metaseq/pull/429#discussion_r998848249 is an example where learned position still defaults to False, even though all of our runs now default this to True in our sweep scripts. These shouldn't be set at all and left to users to define each time (until we sort out our config pains).