Make coca and HF work together

[gpucce]

@ae86280 the HF support is indeed not good for coca yet this PR is to address this and also fix #445
@fedshyvana this will also remove the two unused args #434 while adding options for HF
@vturrisi this is the PR where I plan to add the option to output the raw image tokens in CoCa #458

I should be able to work on it in the next days and fix this. So far there is nothing much.

High level Todos are:

• add causal masking for roberta
• add gpt2 as decoder
• adjust configs
• add option to output image tokens in coca visual encoder

[jqyin]

is there still a plan to support coca + HF models ?