Fix type in gpt2 config docstring
This PR corrects the type of the field `embd_pdrop` in the docstring of `configuration_gpt2.py`, the field should be a float not an int, like the default value `0.1` suggests.


## Who can review?
Documentation: @sgugger, @stevhliu and @MKhalusova


_The documentation is not available anymore as the PR was closed or merged._