Fix CVE-2020-1747 and CVE-2020-14343

[carmocca]

What does this PR do?

Fixes #11045

Does your PR introduce any breaking changes? If yes, please list them.

If users were relying on the capabilities of UnsafeLoader when loading the hparams, this will be a breaking change.

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cc @Borda

[tchaton]

LGTM !

[Borda]

can we check that it is fully back compatible with past checkpoints?

[carmocca]

can we check that it is fully back compatible with past checkpoints?

@Borda It isn't if the checkpoint yaml requires arbitrary code execution.