[FEAT] ARIMA analytical gradient

[elephaint]

Adds analytical gradient for ARIMA (rather then relying on internal minimize estimates from scipy's BFGS).

Edit: speedup too limited

[codspeed-hq]

Merging this PR will improve performance by 56.56%

⚡ 2 improved benchmarks
✅ 36 untouched benchmarks

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	test_efficiency[AutoTBATS]	8.6 s	7.2 s	+19.32%
⚡	test_efficiency[AutoRegressive]	154.7 ms	98.8 ms	+56.56%

---

_{Comparing feat/improve_arima (11b3bde) with main (f868f2a)}