[Compile] Add NEON implementation for bf16->fp32 cast (pytorch#134297)

This changes assembly generated for the following routine
```cpp
void bfloat16tofloat(c10::BFloat16* in, float* out) {
        auto tmp0 = at::vec::Vectorized<c10::BFloat16>::loadu(in, 8);
        auto tmp1 = at::vec::convert<float>(tmp0);
        tmp1.store(out);
}
```
from
```asm
bfloat16tofloat(c10::BFloat16*, float*):
0000000000000034        stp     x29, x30, [sp, #-0x10]!
0000000000000038        mov     x29, sp
000000000000003c        sub     x9, sp, #0x90
0000000000000040        and     sp, x9, #0xffffffffffffffe0
0000000000000044        mov     x8, #0x0
0000000000000048        adrp    x9, 0 ; 0x0
000000000000004c        ldr     x9, [x9]
0000000000000050        ldr     x9, [x9]
0000000000000054        str     x9, [sp, #0x88]
0000000000000058        stp     xzr, xzr, [sp, #0x10]
000000000000005c        ldr     q0, [x0]
0000000000000060        str     q0, [sp]
0000000000000064        ldr     q1, [sp, #0x10]
0000000000000068        stp     q0, q1, [sp, #0x20]
000000000000006c        add     x9, sp, #0x40
0000000000000070        add     x10, sp, #0x20
0000000000000074        add     x11, x10, x8
0000000000000078        ldp     d0, d1, [x11]
000000000000007c        shll.4s v0, v0, #16
0000000000000080        shll.4s v1, v1, #16
0000000000000084        stp     q0, q1, [x9], #0x20
0000000000000088        add     x8, x8, #0x10
000000000000008c        cmp     x8, #0x20
0000000000000090        b.ne    0x74
0000000000000094        add     x8, sp, #0x40
0000000000000098        ld1.4s  { v0, v1 }, [x8]
000000000000009c        st1.4s  { v0, v1 }, [x1]
00000000000000a0        ldr     x8, [sp, #0x88]
00000000000000a4        adrp    x9, 0 ; 0x0
00000000000000a8        ldr     x9, [x9]
00000000000000ac        ldr     x9, [x9]
00000000000000b0        cmp     x9, x8
00000000000000b4        b.ne    0xc4
00000000000000b8        mov     sp, x29
00000000000000bc        ldp     x29, x30, [sp], #0x10
00000000000000c0        ret
00000000000000c4        bl      0xc4
```
to
```asm
bfloat16tofloat(c10::BFloat16*, float*):
0000000000000034        ldr     q0, [x0]
0000000000000038        shll.4s v1, v0, #16
000000000000003c        shll2.4s        v2, v0, #16
0000000000000040        st1.4s  { v1, v2 }, [x1]
0000000000000044        ret
```

And as result speeds up `python3 torchchat.py generate stories110M --num-samples 3 --compile --device cpu --dtype bfloat16` from 33 to 90 tokens/sec

Pull Request resolved: pytorch#134297
Approved by: https://github.com/kimishpatel

Author: malfet

aten/src/ATen/cpu/vec/vec256/vec256_convert.h

@@ -223,6 +223,24 @@ struct VecConvert<
 };
 #endif
 
+#if defined(CPU_CAPABILITY_NEON)
+template <>
+struct VecConvert<float, 1, BFloat16, 1> {
+  static inline VectorizedN<float, 1> apply(
+      const VectorizedN<BFloat16, 1>& src) {
+    VectorizedN<float, 1> result;
+    uint16x8_t u16_8 = vld1q_u16(reinterpret_cast<const uint16_t*>(&src[0]));
+    int32x4_t shift = vdupq_n_s32(16);
+    auto u16_low1 = vget_low_u16(u16_8);
+    auto u16_high1 = vget_high_u16(u16_8);
+    float32x4_t f32x4_0 = vreinterpretq_f32_u32(vshlq_u32(vmovl_u16(u16_low1), shift));
+    float32x4_t f32x4_1 = vreinterpretq_f32_u32(vshlq_u32(vmovl_u16(u16_high1), shift));
+    result[0] = {f32x4_0, f32x4_1};
+    return result;
+  }
+};
+#endif
+
 template <typename src_t>
 struct VecConvert<
     float,