Only import magic_wand if sparsity is enabled

[mgoin]

Tested by making sure magic_wand was uninstalled and this code for a dense model runs fine:

from vllm import LLM, SamplingParams
model = LLM("nm-testing/opt-125m-pruned2.4", enforce_eager=True)

Then testing with a sparse model run:

from vllm import LLM, SamplingParams
model = LLM("nm-testing/opt-125m-pruned2.4", sparsity="sparse_w16a16", enforce_eager=True)

output:

...
  File "/home/michael/code/neuralmagic-vllm/vllm/model_executor/weight_utils.py", line 93, in get_sparse_config
    from vllm.model_executor.layers.sparsity import get_sparsity_config
  File "/home/michael/code/neuralmagic-vllm/vllm/model_executor/layers/sparsity/__init__.py", line 6, in <module>
    raise ValueError(
ValueError: magic_wand is not available and required for sparsity support. Please install it with `pip install magic_wand`

[tlrmchlsmth]

LGTM