fix incorrect codecov reports (#6553)

As discussed at https://github.com/huggingface/transformers/issues/6317 codecov currently sends an invalid report when it fails to find a code coverage report for the base it checks against, so this gets fixed by:

-  require_base: yes        # don't report if there is no base coverage report

let's add this for clarity, this supposedly is already the default.

-  require_head: yes        # don't report if there is no head coverage report 

and perhaps no point reporting on doc changes as they don't make any difference and it just generates noise:

-  require_changes: true    # only comment if there was change in coverage

codecov.yml

@@ -4,3 +4,7 @@ coverage:
       default:
         informational: true
     patch: off
+comment:
+  require_changes: true    # only comment if there was change in coverage
+  require_head: yes        # don't report if there is no head coverage report
+  require_base: yes        # don't report if there is no base coverage report