[TPU] Doc, fix xla_spawn.py, only preprocess dataset once (#4223)

* [TPU] Doc, fix xla_spawn.py, only preprocess dataset once

* Update examples/README.md

* [xla_spawn] Add `_mp_fn` to other Trainer scripts

* [TPU] Fix: eval dataloader was None

examples/xla_spawn.py

@@ -12,17 +12,13 @@ Inspired by https://github.com/pytorch/pytorch/blob/master/torch/distributed/lau
 
 
 import importlib
-import os
 import sys
 from argparse import REMAINDER, ArgumentParser
+from pathlib import Path
 
 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
@@ -44,7 +40,7 @@ def parse_args():
         "training_script",
         type=str,
         help=(
-            "The full module name to the single TPU training "
+            "The full path to the single TPU training "
             "program/script to be launched in parallel, "
             "followed by all the arguments for the "
             "training script"
@@ -61,7 +57,9 @@ def main():
     args = parse_args()
 
     # Import training_script as a module.
-    mod_name = trim_suffix(os.path.basename(args.training_script), ".py")
+    script_fpath = Path(args.training_script)
+    sys.path.append(str(script_fpath.parent.resolve()))
+    mod_name = script_fpath.stem
     mod = importlib.import_module(mod_name)
 
     # Patch sys.argv