Add raw_ir output for pytorch

[MrSidims]

With the output type a user can specify their own way to print IR in the code and this output will be respected by the tool. Example use case: import torch
import torch.nn as nn
from torch_mlir import fx
from torch_mlir.compiler_utils import run_pipeline_with_repro_report from torch_mlir.fx import OutputType

class MyModel(nn.Module):
    def __init__(self):
        super().__init__()
        self.linear = nn.Linear(4, 4)

    def forward(self, x):
        return torch.relu(self.linear(x))

model = MyModel()
example_input = torch.randn(4, 4)
module = fx.export_and_import(model, example_input, output_type=OutputType.LINALG_ON_TENSORS) print(module)

This patch also brings few modifications to README.