Investigate fp16 exp tolerances on XNNPACK

### 🐛 Describe the bug

The XNNPACK fp16 exp implementation appears to differ significantly from eager mode, even by fp16 standards. Tolerances can be in excess of 1e-2. We should investigate this to better understand the tolerances and check correctness.

### Versions

N/A

cc @digantdesai @mcr229 @cbilgin