Improve training speed by 30%~40%

[hbb1]

Fusing some operators, lead to better training speed. Note that all hyper-parameters and setting are kept the same.

Test on the whole DTU dataset with one RTX3090.

	CD	Time
3DGS	1.96	11.2min
2DGS (paper)	0.80	18.2min
2DGS (This)	0.79	10.9min

Tested on a NeRF Synthetic Chair with one RTX3090

EXP	PSNR	Time
2DGS (old)	35.35	12 min
2DGS (old T_precomp)	35.35	21 min
2DGS (This)	35.20	7 min 46s
2DGS (This T_precomp)	35.35	14min 50s

Tested on MipNeRF360 garden scene

EXP	PSNR	Time
2DGS (old)	26.89	50 min 56s
2DGS (This)	26.84	31 min 02s