Tpu trainer (#4146)

* wip

* wip

* a last wip

* Better logging when using TPUs

* Correct argument name

* Tests

* fix

* Metrics in evaluation

* Update src/transformers/training_args.py

* [tpu] Use launcher script instead

* [tpu] lots of tweaks

* Fix formatting

Co-authored-by: Julien Chaumond <chaumond@gmail.com>

examples/xla_spawn.py

@@ -0,0 +1,74 @@
+"""
+A simple launcher script for TPU training
+
+Inspired by https://github.com/pytorch/pytorch/blob/master/torch/distributed/launch.py
+
+::
+    >>> python xla_spawn.py --num_cores=NUM_CORES_YOU_HAVE
+               YOUR_TRAINING_SCRIPT.py (--arg1 --arg2 --arg3 and all other
+               arguments of your training script)
+
+"""
+
+
+import importlib
+import os
+import sys
+from argparse import REMAINDER, ArgumentParser
+
+import torch_xla.distributed.xla_multiprocessing as xmp
+
+
+def trim_suffix(s: str, suffix: str):
+    return s if not s.endswith(suffix) or len(suffix) == 0 else s[: -len(suffix)]
+
+
+def parse_args():
+    """
+    Helper function parsing the command line options
+    @retval ArgumentParser
+    """
+    parser = ArgumentParser(
+        description=(
+            "PyTorch TPU distributed training launch "
+            "helper utility that will spawn up "
+            "multiple distributed processes"
+        )
+    )
+
+    # Optional arguments for the launch helper
+    parser.add_argument("--num_cores", type=int, default=1, help="Number of TPU cores to use (1 or 8).")
+
+    # positional
+    parser.add_argument(
+        "training_script",
+        type=str,
+        help=(
+            "The full module name to the single TPU training "
+            "program/script to be launched in parallel, "
+            "followed by all the arguments for the "
+            "training script"
+        ),
+    )
+
+    # rest from the training program
+    parser.add_argument("training_script_args", nargs=REMAINDER)
+
+    return parser.parse_args()
+
+
+def main():
+    args = parse_args()
+
+    # Import training_script as a module.
+    mod_name = trim_suffix(os.path.basename(args.training_script), ".py")
+    mod = importlib.import_module(mod_name)
+
+    # Patch sys.argv
+    sys.argv = [args.training_script] + args.training_script_args + ["--tpu_num_cores", str(args.num_cores)]
+
+    xmp.spawn(mod._mp_fn, args=(), nprocs=args.num_cores)
+
+
+if __name__ == "__main__":
+    main()