Add uq module (with trainer)

[ziqi-ma]

Initial version of integrating uq module. Added:

• models/uqno.py which is the main uqno object
• examples/UQNO_Darcy.py which is a demonstration of how uqno should be used e2e
• examples/utils.py which is helper function for UQNO_Darcy.py (this can be reorganized to a separate folder)
• losses/data_losses.py PointwiseQuantileLoss which is needed to train pre-calibration residual NO
• changes to the layers/spectral_convolution and fno_block which is unfortunate - I ran my code based on an earlier version of the main repo - tried merging with the current one and the current version is the minimal discrepancy I can get.
• training/trainer.py train_pointwise_err for training with pointwise quantile loss

Darcy data (defined in download_darcy_data in examples/utils.py L90):
"https://drive.google.com/uc?export=download&id=1uQv1geg3HTZjtCS0reh9TMW0EBvvm7xm" (a)
"https://drive.google.com/uc?export=download&id=1zIo5wVhvVJOaufXlJ8uDVpXm1qncqseL" (u)