[BACKEND-2] README Quick Start has asymmetric DTensor and Megatron GRPO commands

## Problem

In the README Quick Start, the `Run GRPO (DTensor)` command does not include a config file, while `Run GRPO (Megatron)` does. This implies to new users that DTensor is the default and requires no configuration, which is misleading.

## Suggested fix

Make the two commands symmetric: either include an explicit config for DTensor or add an inline comment explaining that DTensor auto-selects without one.