Siammask tracker as DL serverless function

[nmanovic]

Motivation and context

Add support of tracker serverless functions into Lambda Manager. Also add SiamMask tracker as a DL serverless function.
The whole idea is trivial. To transform a tracker to DL serverless function it should return its state on each call. In the beginning the state is None and we send shape instead to initialize the tracker. After that it is necessary to send state for each new call of the function to continue the tracking procedure.

An idea for the future: now we use requests only to submit data to DB. It will be great if we have two types of requests: online and offline. Online requests will send results directly to UI at the end. Offline requests will send results to DB as now.

Note: it seems it will not be easy to convert other trackers to DL serverless functions because they hide own state usually. Every time need to implement encode_state and decode_state functions to serialize/deserialize a state to/from JSON. It should not be a problem for python native trackers because python cannot truly incapsulate anything but it is necessary to understand what is a state for a tracker.

Note: it was pain to integrate conda to docker. It is a good example that implementation of a DL serverless function can be non-trivial.

How has this been tested?

Manually. I used model_handler.py from the PR with demo.py from SiamMask GitHub repo and saw that I got the same tracking results.

Checklist

• I submit my changes into the develop branch
• I have added description of my changes into CHANGELOG file
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues (read github docs)
• I have increased versions of npm packages if it is necessary (cvat-canvas,
  cvat-core, cvat-data and cvat-ui)

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.
• I have updated the license header for each file (see an example below)

# Copyright (C) 2020 Intel Corporation
#
# SPDX-License-Identifier: MIT

[nmanovic]

@azhavoro , could you please look at the PR? I see that we have a problem with build. Could you please investigate and fix?

[coveralls]

Pull Request Test Coverage Report for Build 6800

• 0 of 2 (0.0%) changed or added relevant lines in 1 file are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.02%) to 68.933%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
cvat/apps/lambda_manager/views.py	0	2	0.0%

Files with Coverage Reduction	New Missed Lines	%
cvat/apps/engine/media_extractors.py	2	76.99%

Totals
Change from base Build 6798:	-0.02%
Covered Lines:	11150
Relevant Lines:	15668

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💛 - Coveralls