[WIP] Adding config setting to disable singleton cache

[brimoor]

Example usage

import fiftyone as fo

fo.config.singleton_cache = False

dataset = fo.Dataset()
sample = fo.Sample(filepath="image.jpg")

dataset.add_sample(sample)
print(sample.in_dataset)  # True

also_sample = dataset.first()
print(sample is also_sample)  # False

also_dataset = fo.load_dataset(dataset.name)
print(also_dataset is dataset)  # False

TODO

When singleton caching is disabled, there are a variety of places where patterns like this are used where the reload() is now always unnecessary:

dataset = fo.load_dataset(name)
dataset.reload()  # reload in case we got a singleton rather than a refresh load from the database

We should probably refactor those calls into a single method call so that unnecessary reloads can be skipped:

# Reload the dataset (only) if a singleton instance was returned
dataset = fo.load_dataset(name, reload=True)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (8c018ed) 15.85% compared to head (9ad6b78) 15.85%.
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  Coverage    15.85%   15.85%           
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  Files          731      731           
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  Branches      1093     1093           
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