Fix merge_fp32_inputs_into_fp16 with no partition (#38594)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/38594
By default, we don't have parition name, so previous impl will fail to rewire the input into the split-convert output. It's usually a hidden perf issue instead of a correctness issue.
Test Plan:
Enhanced
```
buck test glow/fb/test:test_merge_inputs_nnpi_fp16nnpi
```
Reviewed By: tracelogfb
Differential Revision: D21608439
fbshipit-source-id: d72b06500a3b84f6747aa77cf9fd8754a4ff1195
