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/testing_utils.py

@@ -297,15 +297,15 @@ class CaptureLogger:
 
     Example:
 
-    from transformers import logging
-    from transformers.testing_utils import CaptureLogger
+    >>> from transformers import logging
+    >>> from transformers.testing_utils import CaptureLogger
 
-    msg = "Testing 1, 2, 3"
-    logging.set_verbosity_info()
-    logger = logging.get_logger("transformers.tokenization_bart")
-    with CaptureLogger(logger) as cl:
-        logger.info(msg)
-    assert cl.out, msg+"\n"
+    >>> msg = "Testing 1, 2, 3"
+    >>> logging.set_verbosity_info()
+    >>> logger = logging.get_logger("transformers.tokenization_bart")
+    >>> with CaptureLogger(logger) as cl:
+    ...     logger.info(msg)
+    >>> assert cl.out, msg+"\n"
     """
 
     def __init__(self, logger):