Added template and adhere to standard coding practice

[karan6181]

Description

• Added a pull request template
  • Reference: https://github.com/mosaicml/composer/blob/dev/.github/PULL_REQUEST_TEMPLATE.md
  • Reference: https://github.com/aws/sagemaker-python-sdk/blob/master/.github/PULL_REQUEST_TEMPLATE.md
• Added a CODEOWNERS file
  • Reference: https://github.com/mosaicml/composer/blob/dev/CODEOWNERS
• Added a CODE_OF_CONDUCT file
  • Reference: https://github.com/mosaicml/composer/blob/dev/CODE_OF_CONDUCT.md
• Added a MANIFEST.in file
  • Reference: https://github.com/mosaicml/composer/blob/dev/MANIFEST.in
• Added STYLE_GUIDE file and modified it based on Streaming repo
  • Reference: https://github.com/mosaicml/composer/blob/dev/STYLE_GUIDE.md
• Added yamllint to pre-commit-config to ensure linting runs on yaml files
• Added __all__ to all public modules according to coding guidelines..
• Convert relative import to absolute import according to coding guidelines.

[bandish-shah]

👍 looks great!