Trainer (#3800)

* doc

* [tests] Add sample files for a regression task

* [HUGE] Trainer

* Feedback from @sshleifer

* Feedback from @thomwolf + logging tweak

* [file_utils] when downloading concurrently, get_from_cache will use the cached file for subsequent processes

* [glue] Use default max_seq_length of 128 like before

* [glue] move DataTrainingArguments around

* [ner] Change interface of InputExample, and align run_{tf,pl}

* Re-align the pl scripts a little bit

* ner

* [ner] Add integration test

* Fix language_modeling with API tweak

* [ci] Tweak loss target

* Don't break console output

* amp.initialize: model must be on right device before

* [multiple-choice] update for Trainer

* Re-align to 827d6d6ef0

src/transformers/file_utils.py

@@ -456,6 +456,11 @@ def get_from_cache(
     lock_path = cache_path + ".lock"
     with FileLock(lock_path):
 
+        # If the download just completed while the lock was activated.
+        if os.path.exists(cache_path) and not force_download:
+            # Even if returning early like here, the lock will be released.
+            return cache_path
+
         if resume_download:
             incomplete_path = cache_path + ".incomplete"
 
@@ -496,3 +501,50 @@ def get_from_cache(
             json.dump(meta, meta_file)
 
     return cache_path
+
+
+class cached_property(property):
+    """
+    Descriptor that mimics @property but caches output in member variable.
+
+    From tensorflow_datasets
+
+    Built-in in functools from Python 3.8.
+    """
+
+    def __get__(self, obj, objtype=None):
+        # See docs.python.org/3/howto/descriptor.html#properties
+        if obj is None:
+            return self
+        if self.fget is None:
+            raise AttributeError("unreadable attribute")
+        attr = "__cached_" + self.fget.__name__
+        cached = getattr(obj, attr, None)
+        if cached is None:
+            cached = self.fget(obj)
+            setattr(obj, attr, cached)
+        return cached
+
+
+def torch_required(func):
+    # Chose a different decorator name than in tests so it's clear they are not the same.
+    @wraps(func)
+    def wrapper(*args, **kwargs):
+        if is_torch_available():
+            return func(*args, **kwargs)
+        else:
+            raise ImportError(f"Method `{func.__name__}` requires PyTorch.")
+
+    return wrapper
+
+
+def tf_required(func):
+    # Chose a different decorator name than in tests so it's clear they are not the same.
+    @wraps(func)
+    def wrapper(*args, **kwargs):
+        if is_tf_available():
+            return func(*args, **kwargs)
+        else:
+            raise ImportError(f"Method `{func.__name__}` requires TF.")
+
+    return wrapper