`load_old_state_dict` of `DiffusionModelUNet` is wrong

[yiheng-wang-nv]

Describe the bug
some weights are not handled.

To Reproduce
Using monai-generative==0.2.2 and dev branch of MONAI to compare:

from generative.networks.nets import DiffusionModelUNet
from monai.networks.nets.diffusion_model_unet import DiffusionModelUNet as MonaiDiffusionModelUNet
import torch

input_params = {
    "spatial_dims": 2,
    "in_channels": 1,
    "out_channels": 1,
    "num_channels": [32, 64, 128, 256],
    "attention_levels": [False, True, True, True],
    "num_head_channels": [0, 32, 32, 32],
    "num_res_blocks": 2,
}

old_network = DiffusionModelUNet(**input_params)

new_params = input_params.copy()
new_params.pop("num_channels")
new_params["channels"] = input_params["num_channels"]
new_params["include_fc"] = False
new_params["use_combined_linear"] = False

new_network = MonaiDiffusionModelUNet(**new_params)

for k in new_network.state_dict().keys():
    if "to_q.weight" in k:
        print(k)

for k in old_network.state_dict().keys():
    if "to_q.weight" in k:
        print(k)

new network has layers like:

down_blocks.1.attentions.0.attn.to_q.weight
down_blocks.1.attentions.1.attn.to_q.weight

old network has layers like:

down_blocks.1.attentions.0.to_q.weight
down_blocks.1.attentions.1.to_q.weight

However, the load_old_state_dict function does not handle these layers

[yiheng-wang-nv]

this bug is also related to bundle update: Project-MONAI/model-zoo#621

[yiheng-wang-nv]

I can handle this issue (already solved locally) cc @KumoLiu @ericspod