Feature Request: Enhanced Cross-Datacenter Training UX

## Summary

Request improved user experience for multi-datacenter LLM training via dedicated CLI options, documentation, and example configs. The HSDP foundation exists in Megatron Core v0.11.0, but users must manually configure complex DeviceMesh setups without guidance.

## Motivation

Multi-datacenter training is becoming essential for scaling. The [NVIDIA blog](https://developer.nvidia.com/blog/turbocharge-llm-training-across-long-haul-data-center-networks-with-nvidia-nemo-framework/) demonstrates 96% scaling efficiency training a 340B model across data centers ~1000 km apart. However:

- **High configuration complexity** - Users must manually configure HSDP with DeviceMesh, understand `dp_outer`/`dp_shard` relationships, and tune communication parameters for WAN environments
- **Missing documentation** - "N/S connection" mentioned in CHANGELOG v0.11.0 but never explained; no user guide or examples for cross-datacenter setups
- **No bandwidth/latency-aware tuning** - No automatic adjustment of chunk sizes or communication strategies based on network topology
- **Limited observability** - No metrics distinguishing inter-DC vs intra-DC communication patterns

## Current State

Megatron Core has HSDP (Hybrid Sharded Data Parallel) implementation in `megatron/core/distributed/fsdp/` and basic CLI but lacks user-friendly abstractions for common cross-datacenter deployment patterns.

## Ask

1. **New CLI arguments** for intuitive multi-datacenter configuration

2. **Documentation** - User guide at `docs/user-guide/features/multi_datacenter_training.md` explaining architecture, configuration, and tuning

3. **Example scripts** - `examples/cross_datacenter/` with working configurations for common setups (2-DC, multi-DC with HSDP)

4. **Monitoring** - wandb metrics for inter-DC vs intra-DC communication breakdown

## References

- [NVIDIA Blog: Turbocharge LLM Training Across Long-Haul Data Center Networks](https://developer.nvidia.com/blog/turbocharge-llm-training-across-long-haul-data-center-networks-with-nvidia-nemo-framework/)
- [Megatron-FSDP README](https://github.com/NVIDIA/Megatron-LM/tree/main/megatron/core/distributed/fsdp/src)