Set token_dtype_code init value in GPTDatasetConfig to fix CI

[asolergi-nv]

What does this PR do ?

NeMo CI was crashing in a mbridge test because the __post_init__ method was not being called correctly. This fix protects against this type of errors.

Contribution process

flowchart LR
    A[Pre-checks] --> B[PR Tests]
    subgraph Code Review/Approval
        C1[Expert Review] --> C2[Final Review]
    end
    B --> C1
    C2 --> D[Merge]

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Pre-checks

• I want this PR in a versioned release and have added the appropriate Milestone (e.g., Core 0.8)
• I have added relevant unit tests
• I have added relevant functional tests
• I have added proper typing to my code Typing guidelines
• I have added relevant documentation
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[asolergi-nv]

/ok to test 94830c6

[Phlip79]

Please update description.

[asolergi-nv]

/ok to test 7e05aff

[ko3n1g]

/ok to test 937d421

[ko3n1g]

/ok to test e895adc