Extend the shared samplers to represent the non-openpi conventions Martino
catalogued, without changing existing openpi behavior:
- sample_noise: add distribution="uniform" for evo1's rand*2-1 noise.
- sample_time_beta: add complement flag (groot/wall_x forward t=(1-beta)*s)
and optional clamp_min/clamp_max (evo1 Beta(2,2) clamped to [0.02, 0.98]);
scale/offset now default to the identity mapping.
- sample_beta: build the Beta distribution via a cached, CPU-side helper
(groot convention) so it is constructed once per (alpha, beta).
Adds tests for uniform noise, complement/clamp timesteps, and caching.
Co-authored-by: Cursor <cursoragent@cursor.com>
Martino Russi