Fix clip_qk for virtual pipeline size > 1

[juntaowww]

What does this PR do ?

Add a None check for current_max_attn_logits in clip_qk.

When virtual pipeline is enabled with size > 1, in the first iteration step, only some model chunks have initialized the current_max_attn_logits, other chunks remain with current_max_attn_logits = None, which would lead to a TypeError when all_reduce is called in the following step.

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[yuzhongw-nvidia]

LGTM. Thanks.

[yaox12]

/ok to test b6a88bd

[yaox12]

/ok to test a87b4d4

[yaox12]

/ok to test 0ae7aa6