Add the funtion to plot training and val logs like analyze_logs.py in mmdetection

[caodroid]

Describe the feature

hope the the funtion to plot curve of training and val logs will be implement in mmsegmentation, it is really important to visualize the logs. thank you!
Motivation
A clear and concise description of the motivation of the feature.
In the evaluation of the trained model , the training and val logs , like loss, acc, mIoU, is important and persuasive, if the logs could be visualized ,it will help users to get a more conprehensive understanding of the performence of their models.
Ex1. It is inconvenient when [....].
Ex2. There is a recent paper [....], which is very helpful for [....].

Related resources
If there is an official code release or third-party implementations, please also provide the information here, which would be very helpful.
the code is implemented as analyze_logs.py in mmdetection in your another toolkit, hope it will be implement in mmsegmentation. thank you. https://github.com/open-mmlab/mmdetection/blob/master/tools/analyze_logs.py

Additional context
Add any other context or screenshots about the feature request here.
If you would like to implement the feature and create a PR, please leave a comment here and that would be much appreciated.

[xvjiarui]

Hi @caodroid
We may add it in the next release. You are also welcome to create a PR to add it.

[xvjiarui]

Fixed by #426