  This work targets to merge various Vision Transformers (ViTs) trained on
different tasks (i.e., datasets with different object categories) or domains
(i.e., datasets with the same categories but different environments) into one
unified model, yielding still good performance on each task or domain. Previous
model merging works focus on either CNNs or NLP models, leaving the ViTs
merging research untouched. To fill this gap, we first explore and find that
existing model merging methods cannot well handle the merging of the whole ViT
models and still have improvement space. To enable the merging of the whole
ViT, we propose a simple-but-effective gating network that can both merge all
kinds of layers (e.g., Embedding, Norm, Attention, and MLP) and select the
suitable classifier. Specifically, the gating network is trained by unlabeled
datasets from all the tasks (domains), and predicts the probability of which
task (domain) the input belongs to for merging the models during inference. To
further boost the performance of the merged model, especially when the
difficulty of merging tasks increases, we design a novel metric of model weight
similarity, and utilize it to realize controllable and combined weight merging.
Comprehensive experiments on kinds of newly established benchmarks, validate
the superiority of the proposed ViT merging framework for different tasks and
domains. Our method can even merge beyond 10 ViT models from different vision
tasks with a negligible effect on the performance of each task.
