Change to use relative imports in some files & Add python prompt symbols to example codes (#7202)

* Move 'from transformers' statements to relative imports in some files

* Add python prompt symbols in front of the example codes

* Reformat the code

* Add one missing space

Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

src/transformers/trainer.py

@@ -60,7 +60,7 @@ _use_apex = False
 
 # Check if Pytorch version >= 1.6 to switch between Native AMP and Apex
 if version.parse(torch.__version__) < version.parse("1.6"):
-    from transformers.file_utils import is_apex_available
+    from .file_utils import is_apex_available
 
     if is_apex_available():
         from apex import amp