docs: fix typos in docstrings, comments, and user docs

[mokashang]

What does this PR do?

Fixes a batch of spelling mistakes flagged by codespell across user-facing help text, public docstrings, internal comments, and concept guides. All changes are confined to comments, docstrings, markdown, and --help strings; no behavior or public API changes.

Source code

• src/accelerate/accelerator.py:
  • "wil" → "will" (docstring for even_batches arg of join_uneven_inputs)
  • "especiall" → "especially" (comment about HF Trainer batch_size)
  • "destoying" → "destroying" (Accelerator.end_training docstring)
• src/accelerate/commands/launch.py: "checkoint" → "checkpoint" (--fsdp_cpu_ram_efficient_loading help)
• src/accelerate/utils/dataclasses.py: "checkoint" → "checkpoint" (×2 in FullyShardedDataParallelPlugin docstring + field metadata help)
• src/accelerate/utils/modeling.py: "unexepected" → "unexpected" (load_checkpoint_in_model comment)

Documentation

• docs/source/package_reference/cli.md: "checkoint" → "checkpoint"
• docs/source/usage_guides/compilation.md: "seemless" → "seamless"
• docs/source/usage_guides/low_precision_training.md: "relevent" → "relevant" (×2)
• docs/source/usage_guides/megatron_lm.md: "examle" → "example"
• docs/source/concept_guides/context_parallelism.md: "Parallellism" → "Parallelism" (link text)
• docs/source/concept_guides/sequence_parallelism.md: "Parallellism" → "Parallelism" (×2 link text), "pleae" → "please", "overlayed" → "overlaid"

Benchmarks

• benchmarks/fsdp2/utils.py: "additionaly" → "additionally", "reguire" → "require"
• benchmarks/fsdp2/visualize.py: "roughtly" → "roughly"

Testing

• Re-ran codespell on all changed paths; clean.
• ruff format --check passes on the modified files.
• pytest tests/test_utils.py and pytest tests/test_cli.py (minus the gated transformers-only test) all pass.
• Verified at runtime that the updated docstrings and argparse help strings reflect the corrections.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline?