Reworked Pascal VOC example

[vfdev-5]

Description:

• Simplified files structure
• Horovod backend usage is now possible
• removed MLflow, Plx exp tracking system to simpify the code.

@sdesrozis let me know what do you think

Check list:

• New tests are added (if a new feature is added)
• New doc strings: description and/or example code are in RST format
• Documentation is updated (if required)

[vfdev-5]

@sdesrozis @fco-dv could you please check this PR. Take a look at the new files structure and main.py : https://github.com/pytorch/ignite/tree/110ea08dc69178ffff85d4924a1db48812bde4b0/examples/references/segmentation/pascal_voc2012
Feedback is appreciated.

[vfdev-5]

Note:

• Update README.md ref example section / features
• Update docs / examples / ref example section / features

[sdesrozis]

@vfdev-5 what an improvement ! I'm fan of what you did. Next step is to have this example as a standalone project in the organization repo ?