ElectraForCausalLM
* Add ElectraForCausalLM and cover some basic tests & need to fix a few tests * Fix bugs * make style * make fix-copies * Update doc * Change docstring to markdown format * Remove redundant update_keys_to_ignore
* First pass * Apply suggestions from code review * Apply suggestions from code review Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>