!12348 Fix SequentialSampler issue

From: @mahdirahmanihanzaki
Reviewed-by: 
Signed-off-by:

mindspore/ccsrc/minddata/dataset/engine/datasetops/source/sampler/sequential_sampler.cc

@@ -113,13 +113,11 @@ int64_t SequentialSamplerRT::CalculateNumSamples(int64_t num_rows) {
   int64_t num_samples = (num_samples_ > 0) ? std::min(child_num_rows, num_samples_) : child_num_rows;
   // For this sampler we need to take start_index into account. Because for example in the case we are given n rows
   // and start_index != 0 and num_samples >= n then we can't return all the n rows.
-  if (child_num_rows - (start_index_ - current_id_) <= 0) {
+  if (child_num_rows - start_index_ <= 0) {
     return 0;
   }
-  if (child_num_rows - (start_index_ - current_id_) < num_samples)
-    num_samples = child_num_rows - (start_index_ - current_id_) > num_samples
-                    ? num_samples
-                    : num_samples - (start_index_ - current_id_);
+  if (child_num_rows - start_index_ < num_samples)
+    num_samples = child_num_rows - start_index_ > num_samples ? num_samples : num_samples - start_index_;
   return num_samples;
 }
 

tests/ut/python/dataset/test_sampler_chain.py

@@ -70,16 +70,16 @@ def test_numpyslices_sampler_chain():
     # Use 1 statement to add child sampler
     np_data = [1, 2, 3, 4]
     sampler = ds.SequentialSampler(start_index=1, num_samples=2)
-    sampler = sampler.add_child(ds.SequentialSampler(start_index=1, num_samples=2))
+    sampler.add_child(ds.SequentialSampler(start_index=1, num_samples=2))
     data1 = ds.NumpySlicesDataset(np_data, sampler=sampler)
 
     # Verify dataset size
     data1_size = data1.get_dataset_size()
     logger.info("dataset size is: {}".format(data1_size))
-    assert data1_size == 4
+    assert data1_size == 1
 
     # Verify number of rows
-    assert sum([1 for _ in data1]) == 4
+    assert sum([1 for _ in data1]) == 1
 
     # Verify dataset contents
     res = []