fix(smolvla2): align flow_loss_weight default with Pi 0.5 paper's α=10

Pi 0.5 paper §IV.D Eq. (1) sets the loss balance to α=10 between text
CE and flow MSE: actions are the primary output and the flow head
should dominate the gradient signal. SmolVLA2 was defaulting both
weights to 1.0, which inverts that — text CE (~0.5-2.0 nats) ends up
larger than flow MSE (~0.1-1.0), so the action expert gets less
gradient than the LM head despite being the primary task.

Match the paper's split: text_loss_weight=1.0, flow_loss_weight=10.0.
Same as ``pi052`` (the new full reproduction policy).

Also pin the values explicitly in the SLURM launcher so the choice is
visible and overridable per-run rather than buried in the config
default.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

examples/training/smolvla2_hirobot.slurm

@@ -63,6 +63,8 @@ accelerate launch --multi_gpu --num_processes="$NUM_PROCESSES" \
     --policy.compile_model=false \
     --policy.device=cuda \
     --policy.tokenizer_max_length=512 \
+    --policy.text_loss_weight=1.0 \
+    --policy.flow_loss_weight=10.0 \
     --steps="$STEPS" \
     --policy.scheduler_decay_steps="$STEPS" \
     --batch_size="$BATCH_SIZE" \