Adding racing diffusion model

[krishnbera]

PR to incorporate Racing Diffusion Model (RDM) based on the generative process described in Tillman et al. (2020).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Flag	Coverage Δ
unittests	91.08% <100.00%> (+0.15%)	⬆️

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Files with missing lines	Coverage Δ
ssms/config/_modelconfig/racing_diffusion.py	100.00% <100.00%> (ø)

... and 2 files with indirect coverage changes

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