remove example

[chaoming0625]

This pull request removes the examples/006_fsdp_and_optimizer.py file, which contained a comprehensive example of using JAX with mesh sharding and a custom optimizer. The file included various components such as environment setup, data classes, neural network definitions, optimizer implementations, and training routines.

Key changes:

• Removed the entire examples/006_fsdp_and_optimizer.py file, which included:
  • Environment setup for JAX with mesh sharding (os.environ['XLA_FLAGS'] = '--xla_force_host_platform_device_count=8').
  • Data classes for mesh rules and neural network definitions (MeshRules, MLP).
  • Custom optimizer implementation (SGD, SGDState).
  • Training and visualization code (train_step, dataset generation, and plotting).