adding TRANSFORMERS_VERBOSITY env var (#6961)

* introduce TRANSFORMERS_VERBOSITY env var + test + test helpers

* cleanup

* remove helper function

src/transformers/testing_utils.py

@@ -1,4 +1,5 @@
 import inspect
+import logging
 import os
 import re
 import shutil
@@ -270,6 +271,46 @@ class CaptureStderr(CaptureStd):
         super().__init__(out=False)
 
 
+class CaptureLogger:
+    """Context manager to capture `logging` streams
+
+    Args:
+    - logger: 'logging` logger object
+
+    Results:
+        The captured output is available via `self.out`
+
+    Example:
+
+    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"
+    """
+
+    def __init__(self, logger):
+        self.logger = logger
+        self.io = StringIO()
+        self.sh = logging.StreamHandler(self.io)
+        self.out = ""
+
+    def __enter__(self):
+        self.logger.addHandler(self.sh)
+        return self
+
+    def __exit__(self, *exc):
+        self.logger.removeHandler(self.sh)
+        self.out = self.io.getvalue()
+
+    def __repr__(self):
+        return f"captured: {self.out}\n"
+
+
 class TestCasePlus(unittest.TestCase):
     """This class extends `unittest.TestCase` with additional features.
 
@@ -357,3 +398,14 @@ class TestCasePlus(unittest.TestCase):
         for path in self.teardown_tmp_dirs:
             shutil.rmtree(path, ignore_errors=True)
         self.teardown_tmp_dirs = []
+
+
+def mockenv(**kwargs):
+    """this is a convenience wrapper, that allows this:
+
+    @mockenv(USE_CUDA=True, USE_TF=False)
+    def test_something():
+        use_cuda = os.getenv("USE_CUDA", False)
+        use_tf = os.getenv("USE_TF", False)
+    """
+    return unittest.mock.patch.dict(os.environ, kwargs)