feat:support any num_heads for get_alibi_slope (flashinfer-ai#200)

When I was using flashinfer, I encountered that the heads of some models
were not powers of 2. I refer to
**flashinfer/python/tests/alibi_reference.py**, modifies this part of
the C++ code.

include/flashinfer/pos_enc.cuh

@@ -56,8 +56,9 @@ inline std::string PosEncodingModeToString(const PosEncodingMode& pos_encoding_m
 }
 
 __device__ __forceinline__ float get_alibi_slope(uint32_t head_idx, uint32_t num_heads) {
-  // NOTE(Zihao): here we assume that num_heads is a power of 2
-  return math::ptx_exp2(-8. * float(head_idx + 1) / float(num_heads));
+    int n = math::ptx_exp2((int)math::ptx_log2(num_heads));
+    return head_idx < n ? math::ptx_exp2(-8. * float(head_idx + 1) / float(n))
+                        : math::ptx_exp2(-4. * float((head_idx + 1 - n) * 2 - 1) / float(n));
 }
 
 /*!