add tool pytorch2torchscript (open-mmlab#469)

* add tool pytorch2torchscript

* fix the assert message for pytorch version.

Author: sshuair

docs/useful_tools.md

@@ -74,6 +74,18 @@ Description of arguments:
 
 **Note**: This tool is still experimental. Some customized operators are not supported for now.
 
+### Convert to TorchScript (experimental)
+
+We also provide a script to convert model to [TorchScript](https://pytorch.org/docs/stable/jit.html) format. You can use the pytorch C++ API [LibTorch](https://pytorch.org/docs/stable/cpp_index.html) inference the trained model. The converted model could be visualized by tools like [Netron](https://github.com/lutzroeder/netron). Besides, we also support comparing the output results between Pytorch and TorchScript model.
+
+```shell
+python tools/pytorch2torchscript.py ${CONFIG_FILE} --checkpoint ${CHECKPOINT_FILE} --output-file ${ONNX_FILE} [--shape ${INPUT_SHAPE} --verify]
+```
+
+**Note**: It's only support PyTorch>=1.8.0 for now.
+
+**Note**: This tool is still experimental. Some customized operators are not supported for now.
+
 ## Miscellaneous
 
 ### Print the entire config

tools/pytorch2torchscript.py

@@ -0,0 +1,184 @@
+import argparse
+
+import mmcv
+import numpy as np
+import torch
+import torch._C
+import torch.serialization
+from mmcv.runner import load_checkpoint
+from torch import nn
+
+from mmseg.models import build_segmentor
+
+torch.manual_seed(3)
+
+
+def digit_version(version_str):
+    digit_version = []
+    for x in version_str.split('.'):
+        if x.isdigit():
+            digit_version.append(int(x))
+        elif x.find('rc') != -1:
+            patch_version = x.split('rc')
+            digit_version.append(int(patch_version[0]) - 1)
+            digit_version.append(int(patch_version[1]))
+    return digit_version
+
+
+def check_torch_version():
+    torch_minimum_version = '1.8.0'
+    torch_version = digit_version(torch.__version__)
+
+    assert (torch_version >= digit_version(torch_minimum_version)), \
+        f'Torch=={torch.__version__} is not support for converting to ' \
+        f'torchscript. Please install pytorch>={torch_minimum_version}.'
+
+
+def _convert_batchnorm(module):
+    module_output = module
+    if isinstance(module, torch.nn.SyncBatchNorm):
+        module_output = torch.nn.BatchNorm2d(module.num_features, module.eps,
+                                             module.momentum, module.affine,
+                                             module.track_running_stats)
+        if module.affine:
+            module_output.weight.data = module.weight.data.clone().detach()
+            module_output.bias.data = module.bias.data.clone().detach()
+            # keep requires_grad unchanged
+            module_output.weight.requires_grad = module.weight.requires_grad
+            module_output.bias.requires_grad = module.bias.requires_grad
+        module_output.running_mean = module.running_mean
+        module_output.running_var = module.running_var
+        module_output.num_batches_tracked = module.num_batches_tracked
+    for name, child in module.named_children():
+        module_output.add_module(name, _convert_batchnorm(child))
+    del module
+    return module_output
+
+
+def _demo_mm_inputs(input_shape, num_classes):
+    """Create a superset of inputs needed to run test or train batches.
+
+    Args:
+        input_shape (tuple):
+            input batch dimensions
+        num_classes (int):
+            number of semantic classes
+    """
+    (N, C, H, W) = input_shape
+    rng = np.random.RandomState(0)
+    imgs = rng.rand(*input_shape)
+    segs = rng.randint(
+        low=0, high=num_classes - 1, size=(N, 1, H, W)).astype(np.uint8)
+    img_metas = [{
+        'img_shape': (H, W, C),
+        'ori_shape': (H, W, C),
+        'pad_shape': (H, W, C),
+        'filename': '<demo>.png',
+        'scale_factor': 1.0,
+        'flip': False,
+    } for _ in range(N)]
+    mm_inputs = {
+        'imgs': torch.FloatTensor(imgs).requires_grad_(True),
+        'img_metas': img_metas,
+        'gt_semantic_seg': torch.LongTensor(segs)
+    }
+    return mm_inputs
+
+
+def pytorch2libtorch(model,
+                     input_shape,
+                     show=False,
+                     output_file='tmp.pt',
+                     verify=False):
+    """Export Pytorch model to TorchScript model and verify the outputs are
+    same between Pytorch and TorchScript.
+
+    Args:
+        model (nn.Module): Pytorch model we want to export.
+        input_shape (tuple): Use this input shape to construct
+            the corresponding dummy input and execute the model.
+        show (bool): Whether print the computation graph. Default: False.
+        output_file (string): The path to where we store the
+            output TorchScript model. Default: `tmp.pt`.
+        verify (bool): Whether compare the outputs between
+            Pytorch and TorchScript. Default: False.
+    """
+    if isinstance(model.decode_head, nn.ModuleList):
+        num_classes = model.decode_head[-1].num_classes
+    else:
+        num_classes = model.decode_head.num_classes
+
+    mm_inputs = _demo_mm_inputs(input_shape, num_classes)
+
+    imgs = mm_inputs.pop('imgs')
+
+    # replace the orginal forword with forward_dummy
+    model.forward = model.forward_dummy
+    model.eval()
+    traced_model = torch.jit.trace(
+        model,
+        example_inputs=imgs,
+        check_trace=verify,
+    )
+
+    if show:
+        print(traced_model.graph)
+
+    traced_model.save(output_file)
+    print('Successfully exported TorchScript model: {}'.format(output_file))
+
+
+def parse_args():
+    parser = argparse.ArgumentParser(
+        description='Convert MMSeg to TorchScript')
+    parser.add_argument('config', help='test config file path')
+    parser.add_argument('--checkpoint', help='checkpoint file', default=None)
+    parser.add_argument(
+        '--show', action='store_true', help='show TorchScript graph')
+    parser.add_argument(
+        '--verify', action='store_true', help='verify the TorchScript model')
+    parser.add_argument('--output-file', type=str, default='tmp.pt')
+    parser.add_argument(
+        '--shape',
+        type=int,
+        nargs='+',
+        default=[512, 512],
+        help='input image size (height, width)')
+    args = parser.parse_args()
+    return args
+
+
+if __name__ == '__main__':
+    args = parse_args()
+    check_torch_version()
+
+    if len(args.shape) == 1:
+        input_shape = (1, 3, args.shape[0], args.shape[0])
+    elif len(args.shape) == 2:
+        input_shape = (
+            1,
+            3,
+        ) + tuple(args.shape)
+    else:
+        raise ValueError('invalid input shape')
+
+    cfg = mmcv.Config.fromfile(args.config)
+    cfg.model.pretrained = None
+
+    # build the model and load checkpoint
+    cfg.model.train_cfg = None
+    segmentor = build_segmentor(
+        cfg.model, train_cfg=None, test_cfg=cfg.get('test_cfg'))
+    # convert SyncBN to BN
+    segmentor = _convert_batchnorm(segmentor)
+
+    if args.checkpoint:
+        load_checkpoint(segmentor, args.checkpoint, map_location='cpu')
+
+    # convert the PyTorch model to LibTorch model
+    pytorch2libtorch(
+        segmentor,
+        input_shape,
+        show=args.show,
+        output_file=args.output_file,
+        verify=args.verify)