Update vit_det_backbone.py (#1941)

keras_hub/src/models/vit_det/vit_det_backbone.py

@@ -31,7 +31,7 @@ class ViTDetBackbone(Backbone):
         global_attention_layer_indices (list): Indexes for blocks using
             global attention.
         image_shape (tuple[int], optional): The size of the input image in
-            `(H, W, C)` format. Defaults to `(1024, 1024, 3)`.
+            `(H, W, C)` format. Defaults to `(None, None, 3)`.
         patch_size (int, optional): the patch size to be supplied to the
             Patching layer to turn input images into a flattened sequence of
             patches. Defaults to `16`.
@@ -79,7 +79,7 @@ def __init__(
         intermediate_dim,
         num_heads,
         global_attention_layer_indices,
-        image_shape=(1024, 1024, 3),
+        image_shape=(None, None, 3),
         patch_size=16,
         num_output_channels=256,
         use_bias=True,