Add option to use legacy planning metric definition

[wljungbergh]

As described in #166, there is a discrepancy between how planning metrics (L2 and collision rate) are computed in this repo vs previous works, such as VAD and ST-P3.

However, in the paper, these values are compared in the same table, leading to confusion. This PR enables computation of the UniAD metrics using the legacy definition used by VAD/ST-P3, making comparison easier. However, it still defaults to the "original" UniAD way of evaluating. This results in a ranking shift when comparing the methods, see the table below

Method	L2 1s (m)	L2 2s (m)	L2 3s (m)	CR 1s (%)	CR 2s (%)	CR 3s (%)
ST-P3	1.33	2.11	2.90	0.23	0.62	1.27
UniAD (your current metric)	0.51	0.98	1.65	0.10	0.15	0.61
UniAD (legacy ST-P3 metric)	0.42	0.64	0.91	0.07	0.10	0.22
VAD-Tiny	0.46	0.76	1.12	0.21	0.35	0.58
VAD-Base	0.41	0.70	1.05	0.07	0.17	0.41