fix having too many processes and no attribute of '_closed'

1. When initing SummaryRecord, if check failed, self._close is not set,
   which leads to 'SummaryRecord' object has no attribute '_closed'
   AttributeError later on.

2. There may be too many processes for handling summary adapting.
   See issue #I1K6K7

mindspore/train/summary/_event_writer.py

@@ -47,7 +47,7 @@ class EventWriter(Process):
 
     def run(self):
 
-        with Pool() as pool:
+        with Pool(min(cpu_count(), 32)) as pool:
             deq = deque()
             while True:
                 while deq and deq[0].ready():

mindspore/train/summary/summary_record.py

@@ -89,6 +89,8 @@ class SummaryRecord:
                  file_suffix="_MS",
                  network=None):
 
+        self._event_writer, self._closed = None, False
+
         _check_str_by_regular(file_prefix)
         _check_str_by_regular(file_suffix)
         self.log_path = _make_directory(log_dir)
@@ -113,7 +115,6 @@ class SummaryRecord:
         self.suffix = file_suffix
         self.network = network
         self.has_graph = False
-        self._closed = False
 
         # create the summary writer file
         self.event_file_name = get_event_file_name(self.prefix, self.suffix)
@@ -122,8 +123,6 @@ class SummaryRecord:
         except Exception as ex:
             raise RuntimeError(ex)
 
-        self._event_writer = None
-
     def _init_event_writer(self):
         """Init event writer and write metadata."""
         event_writer = EventWriter(self.full_file_name, self.flush_time)