Fixed ALiBi (HabanaAI#254)

Fixed ALiB and [MPT-7B](https://www.databricks.com/blog/mpt-7b) model.
Accuracy results comparing to CPU(collected using
[EleutherAI](https://github.com/EleutherAI/lm-evaluation-harness))

| Tasks          | CPU    | HPU    |
| -------------- | ------ | ------ |
| arc_challenge  | 0.4224 | 0.4189 |
| arc_easy       | 0.6974 | 0.6999 |
| hellaswag      | 0.7603 | 0.7626 |
| lambada_openai | 0.7306 | 0.7326 |
| mmlu           | 0.293  | 0.2925 |
| winogrande     | 0.6851 | 0.6811 |

vllm/attention/backends/habana_attn.py

@@ -108,17 +108,10 @@ def __init__(
         self.v_cache = VLLMKVCache()
         self.num_kv_heads = num_heads if num_kv_heads is None else num_kv_heads
         self.sliding_window = sliding_window
-        self.position_bias = None
         self.alibi_slopes = alibi_slopes
         if alibi_slopes is not None:
-            # FIXME(kzawora): Need a general method to set max_seq_len on
-            # per-model basis.
             alibi_slopes_tensor = torch.tensor(alibi_slopes,
                                                dtype=torch.bfloat16)
-            self.position_bias = _make_alibi_bias(alibi_slopes_tensor,
-                                                  num_kv_heads,
-                                                  alibi_slopes_tensor.dtype,
-                                                  max_seq_len)
             self.alibi_slopes = alibi_slopes_tensor
         assert self.num_heads % self.num_kv_heads == 0
         self.num_queries_per_kv = self.num_heads // self.num_kv_heads
@@ -190,11 +183,13 @@ def forward(
                 assert attn_metadata.attn_bias is not None, \
                         'attn_bias must be set before calling model.forward!'
                 attn_bias = attn_metadata.attn_bias
-                if self.alibi_slopes is not None and \
-                    self.position_bias is not None:
-                    attn_bias.add_(self.position_bias[:, :,
-                                                      -attn_bias.size(2):,
-                                                      -attn_bias.size(3):])
+                if self.alibi_slopes is not None:
+                    position_bias = _make_alibi_bias(self.alibi_slopes,
+                                                     self.num_kv_heads,
+                                                     attn_bias.dtype,
+                                                     attn_bias.shape[-1])
+                    attn_bias = attn_bias.tile((1, self.num_kv_heads, 1, 1))
+                    attn_bias.add_(position_bias)
             else:
                 attn_bias = None