text: Title: [BUG] TinyTimeMixer should not create validation set if validation_split is None
  Body: **Describe the bug**
  <!--
  A clear and concise description of what the bug is.
  -->
  
  `TinyTimeMixerForecaster` always creates a validation set even if `validation_split is None`, in which case the `temporal_train_test_split` defaults the `validation_split` to `0.25` 
  
  Following fixes should be made:
  - create `y_test` only if `validation_split is not None`
  - rename data split: `test` to `eval`
  
  **To Reproduce**
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  ```python
  from sktime.forecasting.ttm import TinyTimeMixerForecaster
  from sktime.datasets import load_airline
  y = load_airline()
  
  forecaster = TinyTimeMixerForecaster(
  	config={"context_length": 10},
  	training_args={
  		"output_dir": "test_output",
  		"report_to": "none",
  	},
  	validation_split=None, # defaults validation_split to 0.25
  )
  forecaster.fit(y, fh=[1, 2, 3])
  y_pred = forecaster.predict()
  ```
  
  **Expected behavior**
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  when `validation_split is None`, `test` should not be created and not passed to the `Trainer`
  
  **Versions**
  <details>
  
  <!--
  Please run the following code snippet and paste the output here:
  
  from sktime import show_versions; show_versions()
  -->
  
  ```
  System:
      python: 3.12.5 | packaged by Anaconda, Inc. | (main, Sep 12 2024, 18:27:27) [GCC 11.2.0]
  executable: /home/geetu/miniconda3/envs/ai/bin/python
     machine: Linux-6.8.0-51-generic-x86_64-with-glibc2.39
  
  Python dependencies:
            pip: 24.2
         sktime: 0.34.0
        sklearn: 1.5.2
         skbase: 0.8.3
          numpy: 1.26.4
          scipy: 1.14.1
         pandas: 2.2.3
     matplotlib: 3.9.2
         joblib: 1.4.2
          numba: None
    statsmodels: None
       pmdarima: None
  statsforecast: None
        tsfresh: None
        tslearn: None
          torch: 2.4.1
     tensorflow: 2.18.0
  ```
  
  </details>
  
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