【MindSpore易点通】如何将PyTorch源码转成MindSpore低阶APIP,并在Ascend芯片上实现单机单卡训练

文章来源：华为云论坛_云计算论坛_开发者论坛_技术论坛-华为云

1 概述
本文将介绍如何将PyTorch源码转换成MindSpore低阶API代码,并在Ascend芯片上实现单机单卡训练。
下图展示了MindSpore高阶API、低阶API和PyTorch的训练流程的区别。

与MindSpore高阶API相同，低阶API训练也需要进行：配置运行信息、数据读取和预处理、网络定义、定义损失函数和优化器。具体步骤同高阶API。
2 构造模型(低阶API)
构造模型时，首先将网络原型与损失函数封装，再将组合的模型与优化器封装，最终组合成一个可用于训练的网络。 由于训练并验证中，需计算在训练集上的精度 ，因此返回值中需包含网络的输出值。

import mindsporefrom mindspore import Modelimport mindspore.nn as nnfrom mindspore.ops import functional as Ffrom mindspore.ops import operations as P
class BuildTrainNetwork(nn.Cell):
    '''Build train network.'''
    def __init__(self, my_network, my_criterion, train_batch_size, class_num):
        super(BuildTrainNetwork, self).__init__()
        self.network = my_network
        self.criterion = my_criterion
        self.print = P.Print()
        # Initialize self.output
        self.output = mindspore.Parameter(Tensor(np.ones((train_batch_size, 
                        class_num)), mindspore.float32), requires_grad=False)
 
    def construct(self, input_data, label):
        output = self.network(input_data)
        # Get the network output and assign it to self.output
        self.output = output
        loss0 = self.criterion(output, label)
        return loss0
class TrainOneStepCellV2(TrainOneStepCell):
    '''Build train network.'''
    def __init__(self, network, optimizer, sens=1.0):
        super(TrainOneStepCellV2, self).__init__(network, optimizer, sens=1.0)
 
    def construct(self, *inputs):
        weights = self.weights
        loss = self.network(*inputs)
        # Obtain self.network from BuildTrainNetwork
        output = self.network.output
        sens = P.Fill()(P.DType()(loss), P.Shape()(loss), self.sens)
        # Get the gradient of the network parameters
        grads = self.grad(self.network, weights)(*inputs, sens)
        grads = self.grad_reducer(grads)
        # Optimize model parameters
        loss = F.depend(loss, self.optimizer(grads))
        return loss, output
    # Construct model
model_constructed = BuildTrainNetwork(net, loss_function, TRAIN_BATCH_SIZE, CLASS_NUM)
model_constructed = TrainOneStepCellV2(model_constructed, opt)

3 训练并验证（低阶API）
和PyTorch中类似，采用低阶API进行网络训练并验证。详细步骤如下：

class CorrectLabelNum(nn.Cell):
 
    def __init__(self):
        super(CorrectLabelNum, self).__init__()
        self.print = P.Print()
        self.argmax = mindspore.ops.Argmax(axis=1)
        self.sum = mindspore.ops.ReduceSum()
 
 
    def construct(self, output, target):
        output = self.argmax(output)
        correct = self.sum((output == target).astype(mindspore.dtype.float32))
        return correct
def train_net(model, network, criterion,
    epoch_max, train_path, val_path,
 
    train_batch_size, val_batch_size,
    repeat_size):
    
    """define the training method"""
    # Create dataset
    ds_train, steps_per_epoch_train = create_dataset(train_path,
        do_train=True, batch_size=train_batch_size, repeat_num=repeat_size)
    ds_val, steps_per_epoch_val = create_dataset(val_path, do_train=False,
                batch_size=val_batch_size, repeat_num=repeat_size)
 
 
    # CheckPoint CallBack definition
    config_ck = CheckpointConfig(save_checkpoint_steps=steps_per_epoch_train,
                                keep_checkpoint_max=epoch_max)
    ckpoint_cb = ModelCheckpoint(prefix="train_resnet_cifar10",
                                directory="./", config=config_ck)
 
 
    # Create dict to save internal callback object's parameters
    cb_params = _InternalCallbackParam()
    cb_params.train_network = model
    cb_params.epoch_num = epoch_max
    cb_params.batch_num = steps_per_epoch_train
    cb_params.cur_epoch_num = 0
    cb_params.cur_step_num = 0
    run_context = RunContext(cb_params)
    ckpoint_cb.begin(run_context)
 
 
    print("============== Starting Training ==============")
    correct_num = CorrectLabelNum()
    correct_num.set_train(False)
    
    for epoch in range(epoch_max):
        print("
Epoch:", epoch+1, "/", epoch_max)
        train_loss = 0
        train_correct = 0
        train_total = 0  
        for _, (data, gt_classes) in enumerate(ds_train):
            model.set_train()
            loss, output = model(data, gt_classes)
            train_loss += loss
            correct = correct_num(output, gt_classes)
            correct = correct.asnumpy()
            train_correct += correct.sum()
            # Update current step number
            cb_params.cur_step_num += 1
            # Check whether to save checkpoint or not
            ckpoint_cb.step_end(run_context)
            
        cb_params.cur_epoch_num += 1
        my_train_loss = train_loss/steps_per_epoch_train
        my_train_accuracy = 100*train_correct/(train_batch_size*
                                steps_per_epoch_train)
        print('Train Loss:', my_train_loss)
        print('Train Accuracy:', my_train_accuracy, '%')
        
        print('evaluating {}/{} ...'.format(epoch + 1, epoch_max))
        val_loss = 0
        val_correct = 0
        for _, (data, gt_classes) in enumerate(ds_val):
            network.set_train(False)
            output = network(data)
            loss = criterion(output, gt_classes)
            val_loss += loss
            correct = correct_num(output, gt_classes)
            correct = correct.asnumpy()
            val_correct += correct.sum()
 
 
        my_val_loss = val_loss/steps_per_epoch_val
        my_val_accuracy = 100*val_correct/(val_batch_size*steps_per_epoch_val)
        print('Validation Loss:', my_val_loss)
        print('Validation Accuracy:', my_val_accuracy, '%')
 
 
    print("--------- trains out ---------")

4 运行脚本
启动命令：
python MindSpore_1P_low_API.py --data_path=xxx --epoch_num=xxx
在开发环境的Terminal中运行脚本，可以看到网络输出结果：

注：由于高阶API采用数据下沉模式进行训练，而低阶API不支持数据下沉训练，因此高阶API比低阶API训练速度快。
性能对比：低阶API: 2000 imgs/sec ；高阶API: 2200 imgs/sec
详细代码请前往MindSpore论坛进行下载：华为云论坛_云计算论坛_开发者论坛_技术论坛-华为云