docs: improve docstring scheduling_ipndm.py (#13198)

Improve docstring scheduling ipndm

Author: delmalih

src/diffusers/schedulers/scheduling_ipndm.py

@@ -31,14 +31,18 @@ class IPNDMScheduler(SchedulerMixin, ConfigMixin):
     Args:
         num_train_timesteps (`int`, defaults to 1000):
             The number of diffusion steps to train the model.
-        trained_betas (`np.ndarray`, *optional*):
+        trained_betas (`np.ndarray` or `List[float]`, *optional*):
             Pass an array of betas directly to the constructor to bypass `beta_start` and `beta_end`.
     """
 
     order = 1
 
     @register_to_config
-    def __init__(self, num_train_timesteps: int = 1000, trained_betas: np.ndarray | list[float] | None = None):
+    def __init__(
+        self,
+        num_train_timesteps: int = 1000,
+        trained_betas: np.ndarray | list[float] | None = None,
+    ):
         # set `betas`, `alphas`, `timesteps`
         self.set_timesteps(num_train_timesteps)
 
@@ -56,21 +60,29 @@ def __init__(self, num_train_timesteps: int = 1000, trained_betas: np.ndarray |
         self._begin_index = None
 
     @property
-    def step_index(self):
+    def step_index(self) -> int | None:
         """
         The index counter for current timestep. It will increase 1 after each scheduler step.
+
+        Returns:
+            `int` or `None`:
+                The index counter for current timestep.
         """
         return self._step_index
 
     @property
-    def begin_index(self):
+    def begin_index(self) -> int | None:
         """
         The index for the first timestep. It should be set from pipeline with `set_begin_index` method.
+
+        Returns:
+            `int` or `None`:
+                The index for the first timestep.
         """
         return self._begin_index
 
     # Copied from diffusers.schedulers.scheduling_dpmsolver_multistep.DPMSolverMultistepScheduler.set_begin_index
-    def set_begin_index(self, begin_index: int = 0):
+    def set_begin_index(self, begin_index: int = 0) -> None:
         """
         Sets the begin index for the scheduler. This function should be run from pipeline before the inference.
 
@@ -169,7 +181,7 @@ def step(
         Args:
             model_output (`torch.Tensor`):
                 The direct output from learned diffusion model.
-            timestep (`int`):
+            timestep (`int` or `torch.Tensor`):
                 The current discrete timestep in the diffusion chain.
             sample (`torch.Tensor`):
                 A current instance of a sample created by the diffusion process.
@@ -228,7 +240,30 @@ def scale_model_input(self, sample: torch.Tensor, *args, **kwargs) -> torch.Tens
         """
         return sample
 
-    def _get_prev_sample(self, sample, timestep_index, prev_timestep_index, ets):
+    def _get_prev_sample(
+        self,
+        sample: torch.Tensor,
+        timestep_index: int,
+        prev_timestep_index: int,
+        ets: torch.Tensor,
+    ) -> torch.Tensor:
+        """
+        Predicts the previous sample based on the current sample, timestep indices, and running model outputs.
+
+        Args:
+            sample (`torch.Tensor`):
+                The current sample.
+            timestep_index (`int`):
+                Index of the current timestep in the schedule.
+            prev_timestep_index (`int`):
+                Index of the previous timestep in the schedule.
+            ets (`torch.Tensor`):
+                The running sequence of model outputs.
+
+        Returns:
+            `torch.Tensor`:
+                The predicted previous sample.
+        """
         alpha = self.alphas[timestep_index]
         sigma = self.betas[timestep_index]
 
@@ -240,5 +275,5 @@ def _get_prev_sample(self, sample, timestep_index, prev_timestep_index, ets):
 
         return prev_sample
 
-    def __len__(self):
+    def __len__(self) -> int:
         return self.config.num_train_timesteps