Speedup import time with lazy import of scipy.stats

[ricardoV94]

This reduces pytensor import time from average 0.9 to 0.45s on my machine

Before:

python -m benchmark_imports pytensor
0.8324 root       pytensor                                
0.3692 project    pytensor.tensor.random                  
0.3689 project    pytensor.tensor.random.rewriting        
0.3649 project    pytensor.tensor.random.rewriting.jax    
0.3640 project    pytensor.tensor.random.basic            
0.3616 dependency scipy.stats                             
0.2857 transitive scipy.stats._stats_py                    from scipy.stats
0.2108 project    pytensor.scalar                         
0.1785 project    pytensor.scalar.math                    
0.1615 dependency scipy.special                           
0.1484 transitive scipy.stats.distributions                from scipy.stats._stats_py
0.1056 project    pytensor.configdefaults                 
0.1020 transitive scipy.stats._continuous_distns           from scipy.stats.distributions
0.0888 dependency numpy                                   
0.0794 transitive scipy.special._support_alternative_backends from scipy.special
0.0791 transitive scipy._lib._array_api                    from scipy.special._support_alternative_backends
0.0767 transitive scipy._lib.array_api_compat.numpy        from scipy._lib._array_api
0.0760 project    pytensor.tensor                         
0.0711 transitive scipy.optimize                           from scipy.stats._stats_py
0.0656 transitive scipy.special._orthogonal                from scipy.special
0.0649 transitive scipy.linalg                             from scipy.special._orthogonal
0.0645 project    pytensor.tensor.rewriting               
0.0477 transitive scipy.linalg._sketches                   from scipy.linalg
0.0474 transitive numpy.f2py                               from scipy._lib.array_api_compat.numpy
0.0470 transitive numpy.f2py.f2py2e                        from numpy.f2py

After:

python -m benchmark_imports pytensor
0.4396 root       pytensor                                
0.1776 project    pytensor.scalar                         
0.1490 project    pytensor.scalar.math                    
0.1348 dependency scipy.special                           
0.0836 project    pytensor.configdefaults                 
0.0766 project    pytensor.tensor                         
0.0733 transitive scipy.special._support_alternative_backends from scipy.special
0.0730 transitive scipy._lib._array_api                    from scipy.special._support_alternative_backends
0.0708 transitive scipy._lib.array_api_compat.numpy        from scipy._lib._array_api
0.0670 project    pytensor.tensor.rewriting               
0.0647 dependency numpy                                   
0.0466 transitive scipy.special._orthogonal                from scipy.special
0.0460 transitive scipy.linalg                             from scipy.special._orthogonal
0.0417 transitive numpy.f2py                               from scipy._lib.array_api_compat.numpy
0.0413 transitive numpy.f2py.f2py2e                        from numpy.f2py
0.0312 transitive scipy.linalg._sketches                   from scipy.linalg
0.0305 transitive scipy.sparse                             from scipy.linalg._sketches
0.0302 transitive numpy.__config__                         from numpy
0.0297 transitive numpy._core                              from numpy.__config__
0.0290 transitive numpy.f2py.crackfortran                  from numpy.f2py.f2py2e
0.0282 project    pytensor.scalar.basic                   
0.0262 project    pytensor.tensor.rewriting.basic         
0.0244 project    pytensor.tensor.random                  
0.0242 transitive numpy.lib                                from numpy
0.0241 project    pytensor.tensor.random.rewriting        

---

📚 Documentation preview 📚: https://pytensor--1268.org.readthedocs.build/en/1268/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.99%. Comparing base (bf628c9) to head (5df5bdc).
Report is 6 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1268   +/-   ##
=======================================
  Coverage   81.99%   81.99%           
=======================================
  Files         188      188           
  Lines       48608    48610    +2     
  Branches     8688     8689    +1     
=======================================
+ Hits        39857    39859    +2     
  Misses       6586     6586           
  Partials     2165     2165           

Files with missing lines	Coverage Δ
pytensor/tensor/random/basic.py	98.86% <100.00%> (+<0.01%)	⬆️

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