Fix inference tests on new segments for DeepARModel and TFTModel

[Mr-Geekman]

Before submitting (must do checklist)

• Did you read the contribution guide?
• Did you update the docs? We use Numpy format for all the methods and classes.
• Did you write any new necessary tests?
• Did you update the CHANGELOG?

Proposed Changes

Set categorical encoder manually, otherwise it is set automatically without option to work with new categories.

Closing issues

Closes #1088.

[github-actions]

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[codecov-commenter]

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[martins0n]

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