Add TAPAS MLM-only models
# What does this PR do?

As requested by #12916, I've converted the TAPAS checkpoints which were pre-trained on masked language modeling (MLM) only. `TapasForMaskedLM` was previously defined, but there were no actual checkpoints available with the language modeling head. They're available now on the hub:

* [google/tapas-large-masklm](https://huggingface.co/google/tapas-large-masklm)
* [google/tapas-base-masklm](https://huggingface.co/google/tapas-base-masklm)
* [google/tapas-medium-masklm](https://huggingface.co/google/tapas-medium-masklm)
* [google/tapas-small-masklm](https://huggingface.co/google/tapas-small-masklm)
* [google/tapas-mini-masklm](https://huggingface.co/google/tapas-mini-masklm)
* [google/tapas-tiny-masklm](https://huggingface.co/google/tapas-tiny-masklm).

It also cleans up the conversion script of TAPAS a bit.

Fixes #12916 
Thanks for this @NielsRogge !