(add) seq2seq example in huggingface/transformers

+import logging
+import os
+from pathlib import Path
+
+import numpy as np
+import pytorch_lightning as pl
+import torch
+from pytorch_lightning.callbacks import EarlyStopping, ModelCheckpoint
+from pytorch_lightning.utilities import rank_zero_only
+
+
+def count_trainable_parameters(model):
+    model_parameters = filter(lambda p: p.requires_grad, model.parameters())
+    params = sum([np.prod(p.size()) for p in model_parameters])
+    return params
+
+
+logger = logging.getLogger(__name__)
+
+
+class Seq2SeqLoggingCallback(pl.Callback):
+    def on_batch_end(self, trainer, pl_module):
+        lrs = {f"lr_group_{i}": param["lr"] for i, param in enumerate(pl_module.trainer.optimizers[0].param_groups)}
+        pl_module.logger.log_metrics(lrs)
+
+    @rank_zero_only
+    def _write_logs(
+        self, trainer: pl.Trainer, pl_module: pl.LightningModule, type_path: str, save_generations=True
+    ) -> None:
+        logger.info(f"***** {type_path} results at step {trainer.global_step:05d} *****")
+        metrics = trainer.callback_metrics
+        trainer.logger.log_metrics({k: v for k, v in metrics.items() if k not in ["log", "progress_bar", "preds"]})
+        # Log results
+        od = Path(pl_module.hparams.output_dir)
+        if type_path == "test":
+            results_file = od / "test_results.txt"
+            generations_file = od / "test_generations.txt"
+        else:
+            # this never gets hit. I prefer not to save intermediate generations, and results are in metrics.json
+            # If people want this it will be easy enough to add back.
+            results_file = od / f"{type_path}_results/{trainer.global_step:05d}.txt"
+            generations_file = od / f"{type_path}_generations/{trainer.global_step:05d}.txt"
+            results_file.parent.mkdir(exist_ok=True)
+            generations_file.parent.mkdir(exist_ok=True)
+        with open(results_file, "a+") as writer:
+            for key in sorted(metrics):
+                if key in ["log", "progress_bar", "preds"]:
+                    continue
+                val = metrics[key]
+                if isinstance(val, torch.Tensor):
+                    val = val.item()
+                msg = f"{key}: {val:.6f}\n"
+                writer.write(msg)
+
+        if not save_generations:
+            return
+
+        if "preds" in metrics:
+            content = "\n".join(metrics["preds"])
+            generations_file.open("w+").write(content)
+
+    @rank_zero_only
+    def on_train_start(self, trainer, pl_module):
+        try:
+            npars = pl_module.model.model.num_parameters()
+        except AttributeError:
+            npars = pl_module.model.num_parameters()
+
+        n_trainable_pars = count_trainable_parameters(pl_module)
+        # mp stands for million parameters
+        trainer.logger.log_metrics({"n_params": npars, "mp": npars / 1e6, "grad_mp": n_trainable_pars / 1e6})
+
+    @rank_zero_only
+    def on_test_end(self, trainer: pl.Trainer, pl_module: pl.LightningModule):
+        return self._write_logs(trainer, pl_module, "test")
+
+
+def get_checkpoint_callback(output_dir, metric):
+    """Saves the best model by validation ROUGE2 score."""
+    if metric == "rouge2":
+        exp = "{val_avg_rouge2:.4f}-{step_count}"
+    elif metric == "bleu":
+        exp = "{val_avg_bleu:.4f}-{step_count}"
+    else:
+        raise NotImplementedError(
+            f"seq2seq callbacks only support rouge2 and bleu, got {metric}, You can make your own by adding to this function."
+        )
+
+    checkpoint_callback = ModelCheckpoint(
+        filepath=os.path.join(output_dir, exp),
+        monitor=f"val_{metric}",
+        mode="max",
+        save_top_k=1,
+        period=0,  # maybe save a checkpoint every time val is run, not just end of epoch.
+    )
+    return checkpoint_callback
+
+
+def get_early_stopping_callback(metric, patience):
+    return EarlyStopping(
+        monitor=f"val_{metric}",
+        mode="max",
+        patience=patience,
+        verbose=True,
+    )