PyTorch convert function for op 'dictconstruct' not implemented

Hi

I'm trying to convert RobertaForMaskedLM into coreml model, but I encountered a runtime error : PyTorch convert function for op 'dictconstruct' not implemented. My code shows as below:

```
from transformers import RobertaTokenizer, RobertaForMaskedLM
import torch

tokenizer = RobertaTokenizer.from_pretrained('roberta-base')
model = RobertaForMaskedLM.from_pretrained('roberta-base')

example_input = torch.randint(1,100,(1,512))

model.cpu()
model.eval()

traced_model = torch.jit.trace(model, example_input ,strict=False)

traced_model.eval()
traced_model.save('./model_RoBERTa.pt')

# coreml conversion
from coremltools.converters.mil import register_torch_op
from coremltools.converters.mil.frontend.torch.ops import _get_inputs
from coremltools.converters.mil.mil import Builder as mb

@register_torch_op
def type_as(context, node):
inputs = _get_inputs(context, node)
context.add(mb.cast(x=inputs[0], dtype='int32'), node.name)

import numpy as np

mlmodel = ct.convert(traced_model, inputs=[ct.TensorType(name="input", shape=example_input.shape, dtype=np.int32)])
```
Add @register_torch_op because this issue: https://github.com/apple/coremltools/issues/960, if not added, "'type_as' not implemented" error will be showed.

environment:
coremltools 4.1
pytorch 1.8.1

Thanks for your help in advance.

Reproduce the report with the provided RobertaForMaskedLM tracing and ct.convert example, starting from coremltools.converters.mil.frontend.torch.ops and the register_torch_op entry point. Done means this example no longer raises “dictconstruct” not implemented during conversion; keep the reported coremltools 4.1 and PyTorch 1.8.1 versions in the reproduction.

Written by the indexing model from the issue text.