PagedAttentionWithALiBi uses wrong dtype for bias

[Yard1]

          This commit breaks PagedAttentionWithALiBi, where the bias from [set_attn_bias](https://github.com/vllm-project/vllm/blob/1117aa1411d9858ea5eef8a81e044379432e6d0e/vllm/model_executor/layers/attention.py#L343) is initialized to a wrong dtype, causing `RuntimeError: invalid dtype for bias - should match query's dtype`

Originally posted by @chu-tianxiang in #971 (comment)