update docs, and small improvements in train

docs/source/multi_gpu_training.mdx

@@ -13,27 +13,9 @@ pip install accelerate
 Or install it with the LeRobot accelerate extra:
 
 ```bash
-pip install lerobot[accelerate]
+pip install -e ".[accelerate]"
 ```
 
-## Configuration (Optional)
-
-You can optionally configure accelerate for your hardware setup by running:
-
-```bash
-accelerate config
-```
-
-This interactive setup will ask you questions about your training environment (number of GPUs, mixed precision settings, etc.) and saves the configuration for future use. For a simple multi-GPU setup on a single machine, you can use these recommended settings:
-
-- Compute environment: This machine
-- Number of machines: 1
-- Number of processes: (number of GPUs you want to use)
-- GPU ids to use: (leave empty to use all)
-- Mixed precision: fp16 or bf16 (recommended for faster training)
-
-**Note:** You can skip this step and specify parameters directly in the launch command (see Option 1 below).
-
 ## Training with Multiple GPUs
 
 You can launch training in two ways:
@@ -64,7 +46,21 @@ accelerate launch \
 
 ### Option 2: Using accelerate config
 
-If you prefer to save your configuration, run `accelerate config` once and then simply launch with:
+If you prefer to save your configuration, you can optionally configure accelerate for your hardware setup by running:
+
+```bash
+accelerate config
+```
+
+This interactive setup will ask you questions about your training environment (number of GPUs, mixed precision settings, etc.) and saves the configuration for future use. For a simple multi-GPU setup on a single machine, you can use these recommended settings:
+
+- Compute environment: This machine
+- Number of machines: 1
+- Number of processes: (number of GPUs you want to use)
+- GPU ids to use: (leave empty to use all)
+- Mixed precision: fp16 or bf16 (recommended for faster training)
+
+Then launch training with:
 
 ```bash
 accelerate launch $(which lerobot-train) \

src/lerobot/scripts/lerobot_train.py

@@ -101,12 +101,10 @@ def update_policy(
 
     if accelerator:
         accelerator.backward(loss)
-        accelerator.unscale_gradients(optimizer=optimizer)
-        grad_norm = torch.nn.utils.clip_grad_norm_(
-            policy.parameters(),
-            grad_clip_norm,
-            error_if_nonfinite=False,
-        )
+        if grad_clip_norm > 0:
+            grad_norm = accelerator.clip_grad_norm_(policy.parameters(), grad_clip_norm)
+        else:
+            grad_norm = torch.tensor(0.0, device=policy.device)
         optimizer.step()
     else:
         grad_scaler.scale(loss).backward()
@@ -198,9 +196,18 @@ def train(cfg: TrainPipelineConfig, accelerator: Callable | None = None):
     torch.backends.cudnn.benchmark = True
     torch.backends.cuda.matmul.allow_tf32 = True
 
+    # Dataset loading synchronization: main process downloads first to avoid race conditions
     if is_main_process:
         logging.info("Creating dataset")
-    dataset = make_dataset(cfg)
+        dataset = make_dataset(cfg)
+    
+    # Wait for main process to finish downloading/caching dataset
+    if accelerator:
+        accelerator.wait_for_everyone()
+    
+    # Now all other processes can safely load the dataset
+    if not is_main_process:
+        dataset = make_dataset(cfg)
 
     # Create environment used for evaluating checkpoints during training on simulation data.
     # On real-world data, no need to create an environment as evaluations are done outside train.py,
@@ -270,6 +277,10 @@ def train(cfg: TrainPipelineConfig, accelerator: Callable | None = None):
         logging.info(f"{cfg.steps=} ({format_big_number(cfg.steps)})")
         logging.info(f"{dataset.num_frames=} ({format_big_number(dataset.num_frames)})")
         logging.info(f"{dataset.num_episodes=}")
+        if accelerator:
+            num_processes = accelerator.num_processes
+            effective_bs = cfg.batch_size * num_processes
+            logging.info(f"Effective batch size: {cfg.batch_size} x {num_processes} = {effective_bs}")
         logging.info(f"{num_learnable_params=} ({format_big_number(num_learnable_params)})")
         logging.info(f"{num_total_params=} ({format_big_number(num_total_params)})")
 
@@ -312,8 +323,10 @@ def train(cfg: TrainPipelineConfig, accelerator: Callable | None = None):
         "dataloading_s": AverageMeter("data_s", ":.3f"),
     }
 
+    # Use effective batch size for proper epoch calculation in distributed training
+    effective_batch_size = cfg.batch_size * (accelerator.num_processes if accelerator else 1)
     train_tracker = MetricsTracker(
-        cfg.batch_size,
+        effective_batch_size,
         dataset.num_frames,
         dataset.num_episodes,
         train_metrics,