Fake Distributed Backend

TorchTitan provides two distinct test modes:

• fake makes every distributed axis fake. Use it for configuration, shape, ownership, and rank-local allocation analysis without real transport.
• real_pp_fake_spmd uses one physical process per pipeline rank. PP traffic is real; data, tensor, context, and expert parallel axes are fake. Use it when pipeline communication and buffer lifetimes must be exercised.

Neither mode replaces a real distributed numerical or performance test. The authoritative user-facing launch contract, including complete command examples and environment-variable semantics, is in docs/debugging.md.

Logical Rank Coordinates

Set NGPU to the logical world size. For pure fake PP, also set: