Add analysis of FP8 + activation checkpointing memory issue

[djmatusz-lambda]

• Identified root cause: FP8 implementation unaware of checkpointing context
• FP8 always saves HP tensors, conflicting with checkpointing memory savings
• Documented issue in float8_linear.py matmul_with_hp_or_float8_args function
• Proposed solution with checkpointing context detection
• Includes implementation plan and expected benefits

[pytorch-bot[bot]]

:link: Helpful Links

:test_tube: See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/ao/3349

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