[WarmupDecayLR] fix log(0) & 1/log(1) bugs (#772)
* fix log(0) & 1/log(1) bugs
* simplify
Co-authored-by: Jeff Rasley <jerasley@microsoft.com>
Co-authored-by: Reza Yazdani <44502768+RezaYazdaniAminabadi@users.noreply.github.com>
Co-authored-by: Cheng Li <pistasable@gmail.com>
