[MLA-1306] avoid copying png data

[chriselion]

Proposed change(s)

Follow-up optimization attempt from #4399

This was basically a wash performance-wise; we can keep if for cleanliness, or just drop the whole idea.

Tested with
mlagents-learn config/ppo/GridWorld.yaml --env=Project/Build/grid --force --num-envs=4
using 1/10 max steps

before
2020-08-26 20:29:32 INFO [stats.py:118] GridWorld. Step: 50000. Time Elapsed: 132.532 s. Mean Reward: -0.195. Std of Reward: 0.952. Training.

"process_pixels": {
    "total": 21.524973707002246,
    "count": 45987,
    "is_parallel": true,
    "self": 4.624946935002843,
    "children": {
        "image_decompress": {
            "total": 16.900026771999404,
            "count": 45987,
            "is_parallel": true,
            "self": 16.900026771999404
        }
    }
}


after
2020-08-26 20:44:20 INFO [stats.py:118] GridWorld. Step: 50000. Time Elapsed: 131.649 s. Mean Reward: -0.504. Std of Reward: 0.878. Training.

"process_pixels": {
    "total": 21.636028249000926,
    "count": 45711,
    "is_parallel": true,
    "self": 4.415242508999466,
    "children": {
        "image_decompress": {
            "total": 17.22078574000146,
            "count": 45711,
            "is_parallel": true,
            "self": 17.22078574000146
        }
    }
}

Useful links (Github issues, JIRA tickets, ML-Agents forum threads etc.)

Types of change(s)

• Bug fix
• New feature
• Code refactor
• Breaking change
• Documentation update
• Other (please describe)

Checklist

• Added tests that prove my fix is effective or that my feature works
• Updated the changelog (if applicable)
• Updated the documentation (if applicable)
• Updated the migration guide (if applicable)

Other comments

[harperj]

Seems odd for this to be _real_tell when it's not used privately / internally.

[chriselion]

Yeah, wasn't sure what to call it. Do you think I should remove it and just use fp.tell directly?

[harperj]

Oh, I was just commenting on the initial underscore indicating it is a "private" method. I think "real_tell" is fine, or maybe "force_tell".

[harperj]

👍 even better