add size-time warning on cpu

[anuragg1209]

This pull request introduces a new time usage tracking feature to the TabPFN library, which includes changes to multiple files to integrate this functionality. The main changes involve adding the TimeUsageTracker class, updating the fit and predict methods of classifiers and regressors to use this tracker, and adding warnings for CPU usage addressing issue #192

Time usage tracking:

• src/tabpfn/model/time_tracker.py: Added the TimeUsageTracker class to track time usage for code segments, with warnings for segments running longer than a specified threshold.

Integration in TabPFNClassifier:

• src/tabpfn/classifier.py: Integrated TimeUsageTracker in the fit and predict_proba methods to start and stop tracking time usage. [1] [2] [3]

Integration in TabPFNRegressor:

• src/tabpfn/regressor.py: Integrated TimeUsageTracker in the fit and predict methods to start and stop tracking time usage. [1] [2] [3]

Constants and utilities:

• src/tabpfn/constants.py: Added constants for CPU size and time warnings.
• src/tabpfn/utils.py: Added a utility function to create the TimeUsageTracker and integrated warnings for CPU usage in the validate_Xy_fit function. [1] [2] [3]