adding MLX backend and example for stable diffusion 2.1

from https://github.com/ml-explore/mlx-examples

backends/mlx/stable_diffusion/config.py

@@ -0,0 +1,48 @@
+# Copyright © 2023 Apple Inc.
+
+from dataclasses import dataclass
+from typing import Optional, Tuple
+
+
+@dataclass
+class AutoencoderConfig:
+    in_channels: int = 3
+    out_channels: int = 3
+    latent_channels_out: int = 8
+    latent_channels_in: int = 4
+    block_out_channels: Tuple[int] = (128, 256, 512, 512)
+    layers_per_block: int = 2
+    norm_num_groups: int = 32
+    scaling_factor: float = 0.18215
+
+
+@dataclass
+class CLIPTextModelConfig:
+    num_layers: int = 23
+    model_dims: int = 1024
+    num_heads: int = 16
+    max_length: int = 77
+    vocab_size: int = 49408
+
+
+@dataclass
+class UNetConfig:
+    in_channels: int = 4
+    out_channels: int = 4
+    conv_in_kernel: int = 3
+    conv_out_kernel: int = 3
+    block_out_channels: Tuple[int] = (320, 640, 1280, 1280)
+    layers_per_block: Tuple[int] = (2, 2, 2, 2)
+    mid_block_layers: int = 2
+    transformer_layers_per_block: Tuple[int] = (1, 1, 1, 1)
+    num_attention_heads: Tuple[int] = (5, 10, 20, 20)
+    cross_attention_dim: Tuple[int] = (1024,) * 4
+    norm_num_groups: int = 32
+
+
+@dataclass
+class DiffusionConfig:
+    beta_schedule: str = "scaled_linear"
+    beta_start: float = 0.00085
+    beta_end: float = 0.012
+    num_train_steps: int = 1000