Remove "`optional`, defaults to :obj:`None`"
There are some docstrings with "`optional`, defaults to :obj:`None`" arguments.
According to @sgugger this should be avoided: https://github.com/huggingface/transformers/pull/11417#discussion_r629320375

PS: I can provide a PR if wanted...
Please go ahead if you want to clean this! A quick serach shows me 17 \`optional\`, defaults to :obj:\`None\` and two `optional`, defaults to None<|||||>ok - see #11703 @sgugger 