Update max context length LlmConfig validation

[jackzhxng]

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[pytorch-bot]

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❌ 2 New Failures

As of commit de4dc62 with merge base 752f6a7 ():

NEW FAILURES - The following jobs have failed:

• pull / unittest / linux / linux-job (gh)
  examples/models/llama/tests/test_ring_attention.py::TestRingAttention::test_sliding_window_attention_custom
• pull / unittest-editable / linux / linux-job (gh)
  examples/models/llama/tests/test_ring_attention.py::TestRingAttention::test_sliding_window_attention_custom

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