The ALG2 for cusparsespmm under performed when feature size is small

## 🐛 Bug
@yzh119 report the ALG2 seems performed badly for cusparsespmm when the feature size is small.
## To Reproduce

Steps to reproduce the behavior:

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## Expected behavior

## Environment

- DGL Version (e.g., 1.0):
- Backend Library & Version (e.g., PyTorch 0.4.1, MXNet/Gluon 1.3):
- OS (e.g., Linux):
- How you installed DGL (`conda`, `pip`, source):
- Build command you used (if compiling from source):
- Python version:
- CUDA/cuDNN version (if applicable):
- GPU models and configuration (e.g. V100):
- Any other relevant information:

## Additional context