Do not force using FlashAttention (#58)

Remove hardcoded selection of operator implementation and use xFormers fMHA dispatcher instead.

dinov2/layers/attention.py

@@ -73,11 +73,7 @@ class MemEffAttention(Attention):
 
         q, k, v = unbind(qkv, 2)
 
-        if attn_bias is not None:
-            self_att_op = fmha.MemoryEfficientAttentionFlashAttentionOp
-        else:
-            self_att_op = None
-        x = memory_efficient_attention(q, k, v, attn_bias=attn_bias, op=self_att_op)
+        x = memory_efficient_attention(q, k, v, attn_bias=attn_bias)
         x = x.reshape([B, N, C])
 
         x = self.proj(x)