Refactor JIT compilation (+NVRTC support)

[lucifer1004]

With this PR, the JIT compilation time is reduced by ~60% when using NVCC, ~80% when using NVRTC w/o PCH, and ~90% when using NVRTC w/ PCH.

Benchmark on Xeon(R) Platinum 8480C + H100 HBM:

m	n	k	compilation time (s)				TFLOPS
			baseline	NVCC	NVRTC	NVRTC+PCH	baseline	NVCC	NVRTC	NVRTC+PCH
64	2112	7168	5.95	2.12	0.96	0.35	154	154	153	153
64	24576	1536	6.43	2.55	1.37	0.78	230	230	231	230
64	32768	512	6.06	2.20	0.99	0.40	180	180	180	180
64	7168	16384	6.11	2.25	1.01	0.43	289	290	289	289
64	4096	7168	5.98	2.15	0.97	0.37	207	207	206	206
64	7168	2048	6.03	2.19	1.01	0.42	188	189	187	188
128	2112	7168	5.98	2.18	0.96	0.37	283	280	281	282
128	24576	1536	6.38	2.56	1.39	0.78	440	435	434	434
128	32768	512	6.00	2.18	0.99	0.40	334	336	330	332
128	7168	16384	6.24	2.40	1.18	0.59	534	534	534	533
128	4096	7168	6.04	2.21	1.03	0.43	370	371	370	371
128	7168	2048	6.19	2.38	1.19	0.62	332	334	331	332
4096	2112	7168	6.09	2.30	1.08	0.49	1053	1058	1113	1114
4096	24576	1536	6.14	2.33	1.10	0.50	1288	1286	1290	1289
4096	32768	512	6.23	2.41	1.21	0.61	913	912	912	911
4096	7168	16384	6.31	2.51	1.27	0.68	1524	1524	1459	1458
4096	4096	7168	6.28	2.47	1.27	0.68	1446	1445	1394	1396
4096	7168	2048	6.18	2.40	1.21	0.61	1240	1243	1234	1237

Note that there is some perf drop when using NVRTC due to a known bug of NVRTC which leads to extra instructions (but in the m=4096,n=2112,k=7168 case, NVRTC version was faster, which was a bit strange). So NVCC is kept as the default compiler for now, while NVRTC can be enabled with extra env var DG_JIT_USE_NVRTC.