Trainer automatically drops unused columns in nlp datasets (#6449)

* Add a classmethod to easily build a Trainer from nlp dataset and metric

* Fix docstrings

* Split train/eval

* Formatting

* Log dropped columns + docs

* Authorize callable activations

* Poc for auto activation

* Be framework-agnostic

* Formatting

* Remove class method

* Remove unnecessary code

src/transformers/file_utils.py

@@ -64,6 +64,14 @@ except (ImportError, AssertionError):
     _tf_available = False  # pylint: disable=invalid-name
 
 
+try:
+    import nlp  # noqa: F401
+
+    _nlp_available = True
+
+except ImportError:
+    _nlp_available = False
+
 try:
     from torch.hub import _get_torch_home
 
@@ -144,6 +152,10 @@ def is_torch_tpu_available():
     return _torch_tpu_available
 
 
+def is_nlp_available():
+    return _nlp_available
+
+
 def is_psutil_available():
     return _psutil_available