Added the framwork in the '_validate()' func call to enable framework tf2

[simonsays1980]

Why are these changes needed?

When using tf2 the _validate() function in the EncoderConfigs' build() method uses always the torch string in the _validate()'s definition. This results in the error shown in #37413

Related issue number

Closes #37413

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• I've included any doc changes needed for https://docs.ray.io/en/master/.
  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
• I've made sure the tests are passing. Note that there might be a few flaky tests, see the recent failures at https://flakey-tests.ray.io/
• Testing Strategy
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  • This PR is not tested :(

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