[Docs] Add sigmoid beta_scheduler to docstrings of relevant Schedulers (#3399)

* Add `sigmoid` beta scheduler to `DDPMScheduler` docstring

* Add `sigmoid` beta scheduler to `RePaintScheduler` docstring

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Co-authored-by: Patrick von Platen <[email protected]>

Author: Laurent2916

src/diffusers/schedulers/scheduling_ddpm.py

@@ -91,7 +91,7 @@ class DDPMScheduler(SchedulerMixin, ConfigMixin):
         beta_end (`float`): the final `beta` value.
         beta_schedule (`str`):
             the beta schedule, a mapping from a beta range to a sequence of betas for stepping the model. Choose from
-            `linear`, `scaled_linear`, or `squaredcos_cap_v2`.
+            `linear`, `scaled_linear`, `squaredcos_cap_v2` or `sigmoid`.
         trained_betas (`np.ndarray`, optional):
             option to pass an array of betas directly to the constructor to bypass `beta_start`, `beta_end` etc.
         variance_type (`str`):

src/diffusers/schedulers/scheduling_repaint.py

@@ -89,7 +89,7 @@ class RePaintScheduler(SchedulerMixin, ConfigMixin):
         beta_end (`float`): the final `beta` value.
         beta_schedule (`str`):
             the beta schedule, a mapping from a beta range to a sequence of betas for stepping the model. Choose from
-            `linear`, `scaled_linear`, or `squaredcos_cap_v2`.
+            `linear`, `scaled_linear`, `squaredcos_cap_v2` or `sigmoid`.
         eta (`float`):
             The weight of noise for added noise in a diffusion step. Its value is between 0.0 and 1.0 -0.0 is DDIM and
             1.0 is DDPM scheduler respectively.