[build] chore: bump MCore main to 07046cc6

[cuichenx]

Advances the matching .main.commit and 3rdparty/Megatron-LM pins from 1e3c0c62fbb79c26d083f506f7da46f5f7add49e to 07046cc6b8b7479ba05e6b47ed25f3f8e6634573 (upstream comparison).

The six upstream commits include MTP acceptance argmax reuse from TE cross entropy (#8143), dev CI cuBLAS updates, upstream test changes, and maintenance metadata. No Bridge compatibility edits are required by inspection. .dev.commit, Bridge dependency metadata, and uv.lock are unchanged; no Bridge tests are quarantined or weakened.

Validation: Linux/CUDA container uv lock and uv lock --check passed (345 packages, no delta); all-file pre-commit, git diff --check, matching main pins and unchanged dev pin passed. Requested CI: standard gates and L0 only on H100 and GB200. All requested gates passed on 2138708998f0fb09ca28b587b3dcb599bf866424: CICD NeMo 38620771316 attempt 1, four unit jobs, 30 H100 and 12 GB200 L0 jobs; configure confirms L1/L2 disabled. Build/release, installation, copyright, docs preview, secrets, links and status checks passed. Automatically merged through the authorized pin-only merge helper.

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