[infra]: MagiHuman checkpoint conversion + push scripts

Two scripts for the daVinci-MagiHuman umbrella HF repo:

- scripts/checkpoint_conversion/convert_magi_human_to_diffusers.py:
  Reference-format -> FastVideo-Diffusers state-dict converter. Supports
  --cast-bf16 with a load-bearing _FP32_KEEP_SUFFIXES allowlist (see the
  accompanying lesson; this was the distill-DiT parity canary at commit
  829f70d of the original PR).
- scripts/checkpoint_conversion/push_magi_human_to_hf.py: Push converted
  weights to the FastVideo/MagiHuman-Diffusers umbrella repo.

Plus the third extracted lesson from PR #1280:
- .agents/lessons/2026-05-07_conversion-cast-bf16-suffix-allowlist.md:
  generic guidance for any DiT converter that ships a global-downcast flag.

Magi-Stack: 7/8

Author: SolitaryThinker

.agents/lessons/2026-05-07_conversion-cast-bf16-suffix-allowlist.md

@@ -0,0 +1,73 @@
+---
+date: 2026-05-07
+experiment: PR #1280 (daVinci-MagiHuman port), distill DiT parity bring-up
+category: porting
+severity: important
+---
+
+# Conversion `--cast-bf16` Needs an FP32-Keep Suffix Allowlist
+
+## What Happened
+
+`scripts/checkpoint_conversion/convert_magi_human_to_diffusers.py --cast-bf16`
+produced a converted distill DiT checkpoint that loaded cleanly, ran end-to-
+end, and emitted reasonable output — but `test_magi_human_distill_parity`
+showed `diff_mean=0.114` against the upstream reference. The base DiT was
+bit-exact with the same conversion script. Only the distill variant
+regressed.
+
+The error was small enough that visual quality looked normal, but large
+enough to fail bit-exact parity. The MagiHuman base + distill DiTs share
+most of their architecture, so a difference that affected only distill was
+counterintuitive.
+
+## Root Cause
+
+`--cast-bf16` was downcasting **all** fp32 tensors to bf16 indiscriminately.
+The base checkpoint and the FastVideo `final_linear` / adapter modules
+require eight specific tensors to remain in fp32:
+
+- LayerNorm `gamma` / `beta` weights for the final residual exit
+- Adapter projection biases
+- A handful of scale parameters in the output projection chain
+
+These tensors participate in chains where bf16 precision causes accumulation
+error large enough to drift the parity check. The base DiT happened to not
+hit those specific chains in the path the test exercised (different
+attention mask shape, different audio interleave); the distill variant did.
+
+## Fix / Workaround
+
+Added `_FP32_KEEP_SUFFIXES` allowlist to
+`convert_magi_human_to_diffusers.py` (commit `829f70d3`) and gated `--cast-
+bf16` on it. Tensors whose state-dict key ends with any allowlisted suffix
+keep their original fp32 dtype regardless of the flag.
+
+Distill DiT parity went from `diff_mean=0.114` (silently wrong) to bit-exact
+in one commit.
+
+## Prevention
+
+1. **Treat `--cast-bf16` as opinionated, not blanket.** Any conversion
+   script that supports a global dtype downcast flag MUST own an explicit
+   allowlist of fp32-keep tensors, documented at the top of the file.
+
+2. **The `add-model-conversion` skill** should enforce two checks for any
+   converter that ships a `--cast-bf16`-style flag:
+   - Run the parity test for **every** variant of the model (base, distill,
+     SR, etc.), not just the headline variant. Different variants exercise
+     different code paths.
+   - Diff the converted checkpoint's dtype map against the upstream
+     reference and assert the allowlist covers every fp32 tensor in the
+     reference.
+
+3. **For MagiHuman specifically**: if you add or rename DiT modules that
+   touch `final_linear`, the adapter, or any LayerNorm in the residual exit
+   path, **check that any fp32-required tensors are covered by
+   `_FP32_KEEP_SUFFIXES`** in the conversion script and re-run
+   `test_magi_human_distill_parity` (it's the canary).
+
+4. The lesson generalizes beyond MagiHuman: any DiT that uses bf16 mixed
+   precision but keeps specific tensors in fp32 (a common pattern with
+   flash-attn-style backends) needs this allowlist for any conversion that
+   downcasts.