Update sequence packing case when dummy PackedSeqParams are used

[mathemakitten]

What does this PR do ?

This line currently creates a 1D tensor of shape (2,) whose values are [0, max_seq_len].
​
For padding to max_seq_len for cudagraphs, we actually want a tensor of shape (max_seq_len,) where the first element is 0 and all others are max_seq_len.

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