docstr: verify error is thrown on invalid backend
docstr_tok: ["verify","error","is","thrown","on","invalid","backend"]
code: def test_prepare_module_compile_invalid_backend(self) -> None:
      """
      verify error is thrown on invalid backend
      """
  
      with self.assertRaises(Exception):
          prepare_module(
              module=torch.nn.Linear(2, 2),
              device=init_from_env(),
              torch_compile_params=TorchCompileParams(backend="foo"),
          )
  
imports: import unittest
  from unittest.mock import patch
  import torch
  from torch.distributed.fsdp import FullyShardedDataParallel
  from torch.distributed.fsdp.fully_sharded_data_parallel import MixedPrecision
  from torch.nn.parallel import DistributedDataParallel
  from torchtnt.utils.env import init_from_env
  from torchtnt.utils.prepare_module import _is_fsdp_module, DDPStrategy, FSDPStrategy, NOOPStrategy, prepare_ddp, prepare_fsdp, prepare_module, TorchCompileParams
  from torchtnt.utils.test_utils import spawn_multi_process
  from torchtnt.utils.version import is_torch_version_geq_1_13, is_torch_version_geq_2_0
label_desc: Use image node_id 22 for calling the PrepareModelTest obj's underlying member method code with example usage: obj.test_prepare_module_compile_invalid_backend() without return types