GPU dependence and non-reproducibility of radiative transfer (small error)

[HajimeKawahara]

During the review of #265 , we found that there is non-reproducibility of radiative transfer results for MODIT and PreMODIT. This can be confirmed by the unit tests:

• exojax/tests/unittests/spec/modit/modit_test.py
• exojax/tests/unittests/spec/premodit/premodit_spectrum_test.py

Differences

• within 0.2 % using A100
• within 2% using GeForce RTX 3080
• intergpu error ~ 3% between A100 and RTX3080

Diff between A100 and RTX3080 by @ykawashima

[HajimeKawahara]

from jax.config import config                                                                                          
config.update("jax_enable_x64", True)

The use of f64 looks avoids this problem.