Add Template of Bug Report (#1321)

* initial issue_template -- bug

* Apply suggestions from code review
Co-authored-by: eqy <eqy@cs.washington.edu>
Co-authored-by: eqy <eqy@cs.washington.edu>

.github/ISSUE_TEMPLATE/bug_report.md

+---
+name: Bug report
+about: Create a report to help us improve apex
+title: ''
+labels: bug
+assignees: ''
+
+---
+
+**Describe the Bug**
+
+**Minimal Steps/Code to Reproduce the Bug**
+<!--
+Please list the *minimal* steps or provide a code snippet for us to be able to reproduce the bug.
+
+A helpful guide on on how to craft a minimal bug report http://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports.
+--> 
+
+**Expected Behavior**
+<!-- A clear and concise description of what you expected to happen. -->
+
+**Environment**
+<!-- OS, version of Python, CUDA, PyTorch; collect these via `python -m torch.utils.collect_env` -->