TF version of the trainer (#4017)

* First commit to add a TF version of the trainer.

* Make the TF trainer closer to what looks the PT trainer

* Refactoring common code between the PT and TF trainer into an util file.

* Some bugfix + better similarity with the PT trainer

* Add missing class in transformers init

* Bugfix over prediction + use classification report instead of simple metrics

* Fix name error

* Fix optimization tests + style

* Apply style

* Several bugfix for multi-gpu training

* Apply style

* Apply style

* Add glue example for the TF trainer

* Several bugix + address the reviews

* Fix on the TF training args file

* Add a debug mode

* Bugfix in utils_ner.py when segment_ids is None

* Apply style

* Apply style

* Add TPU strategy

* Fix selection strategy

src/transformers/trainer_utils.py

@@ -0,0 +1,27 @@
+from typing import Dict, NamedTuple, Optional
+
+import numpy as np
+
+
+class EvalPrediction(NamedTuple):
+    """
+    Evaluation output (always contains labels), to be used
+    to compute metrics.
+    """
+
+    predictions: np.ndarray
+    label_ids: np.ndarray
+
+
+class PredictionOutput(NamedTuple):
+    predictions: np.ndarray
+    label_ids: Optional[np.ndarray]
+    metrics: Optional[Dict[str, float]]
+
+
+class TrainOutput(NamedTuple):
+    global_step: int
+    training_loss: float
+
+
+PREFIX_CHECKPOINT_DIR = "checkpoint"