torch.cuda.is_bf16_supported() defaults to counting pre-Ampere bf16 EMULATION as
supported, so on a T4/V100/RTX 20xx the DiT-training bf16 gates all passed even
though the trainer requires native Ampere-or-newer bf16: /diffusion/info advertised
the DiT precision modes, /diffusion/start's preflight let the run through and freed
resident GPU models, then the trainer child hit the real unsupported bf16 path. The
inference device resolver already fixed this (issue #6658) by gating NVIDIA on
capability major >= 8; the training path never got it. Add a shared
native_bf16_supported() helper (NVIDIA cap major >= 8; ROCm keeps the trustworthy
is_bf16_supported()) and use it in the three DiT bf16 sites -- train_precision_modes,
bf16_unsupported_reason, and the trainer guard -- so a pre-Ampere card is offered
nf4 only and never advertises/evicts-then-fails. Tests now exercise the emulation
case (is_bf16_supported True but capability < 8).
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