[pyspark] hotfix for GPU setup verification issue

[wbo4958]

If the user sets "spark.task.resource.gpu.amount" to be a float value like 1.0 or 0.25, pyspark will throw an exception with the below msg,

  File "/opt/conda/default/lib/python3.10/site-packages/xgboost/spark/core.py", line 657, in _fit
    self._validate_params()
  File "/opt/conda/default/lib/python3.10/site-packages/xgboost/spark/estimator.py", line 230, in _validate_params
    super()._validate_params()
  File "/opt/conda/default/lib/python3.10/site-packages/xgboost/spark/core.py", line 378, in _validate_params
    if not gpu_per_task or int(gpu_per_task) < 1:
ValueError: invalid literal for int() with base 10: '0.25'

This PR is to fix this bug and kind of GPU configuration validating rework.

[wbo4958]

@WeichenXu123 @trivialfis please help to review it.