Support importing SD 2.1

[masahi]

Updated the SD UNet definition to support both v1.5 and v2.1.

Tested with

import tvm
from diffusers import StableDiffusionPipeline, EulerDiscreteScheduler
import web_stable_diffusion.trace as trace
from tvm.relax.frontend import detach_params

# pipe = StableDiffusionPipeline.from_pretrained("runwayml/stable-diffusion-v1-5")

model_id = "stabilityai/stable-diffusion-2-1-base"
scheduler = EulerDiscreteScheduler.from_pretrained(model_id, subfolder="scheduler")
pipe = StableDiffusionPipeline.from_pretrained(model_id, scheduler=scheduler)

unet, params_unet = detach_params(trace.unet_latents_to_noise_pred(pipe, "mps"))

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