Add TensorBoard support (#87)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

ultralytics/yolo/utils/callbacks/tb.py

@@ -0,0 +1,26 @@
+from torch.utils.tensorboard import SummaryWriter
+
+writer = None  # TensorBoard SummaryWriter instance
+
+
+def _log_scalars(scalars, step=0):
+    for k, v in scalars.items():
+        writer.add_scalar(k, v, step)
+
+
+def on_train_start(trainer):
+    global writer
+    writer = SummaryWriter(str(trainer.save_dir))
+    trainer.console.info(f"Logging results to {trainer.save_dir}\n"
+                         f"Starting training for {trainer.args.epochs} epochs...")
+
+
+def on_batch_end(trainer):
+    _log_scalars(trainer.label_loss_items(trainer.tloss, prefix="train"), trainer.epoch)
+
+
+def on_val_end(trainer):
+    _log_scalars(trainer.metrics, trainer.epoch)
+
+
+callbacks = {"on_train_start": on_train_start, "on_val_end": on_val_end, "on_batch_end": on_batch_end}