Fix half precision gemm test accumulation error (#14842)
### Description
Half precision gemm test requirement relaxation
### Motivation and Context
Most CPUs does not support mixed precision accumulation, only mul & add
fuse. As a result, different striding on the K dimension may lead to
rounding error. Accumulation of these rounding error maybe very
significant. So setting an approximation ratio does NOT always work.
What's more, a relaxed test condition may hide real implementation
problem. So this is only a compromised fix.
More rigorous tests require manual efforts:
1. Change the K stride of the kernel under test to be 16.
2. Force the K stride of the fp16 kernel to 16
3. Change the test oracle to be exact match.
4. Pass this test and then change it back :-(.
Co-authored-by: Chen Fu <fuchen@microsoft.com>
