torch/monitor: merge Interval and FixedCount stats (#72009)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/72009
This simplifies the Stats interface by merging IntervalStat and FixedCountStat into a single Stat w/ a specific window size duration and an optional max samples per window. This allows for the original intention of having comparably sized windows (for statistical purposes) while also having a consistent output bandwidth.
Test Plan:
```
buck test //caffe2/test:monitor //caffe2/test/cpp/monitor:monitor
```
Reviewed By: kiukchung
Differential Revision: D33822956
fbshipit-source-id: a74782492421be613a1a8b14341b6fb2e8eeb8b4
(cherry picked from commit 293b94e0b4646521ffe047e5222c4bba7e688464)
