Close matplotlib plots after opening (#4612)

* Close plots

* Replace fig.close() for plt.close()

utils/metrics.py

@@ -178,6 +178,7 @@ class ConfusionMatrix:
             fig.axes[0].set_xlabel('True')
             fig.axes[0].set_ylabel('Predicted')
             fig.savefig(Path(save_dir) / 'confusion_matrix.png', dpi=250)
+            plt.close()
         except Exception as e:
             print(f'WARNING: ConfusionMatrix plot failure: {e}')
 
@@ -308,6 +309,7 @@ def plot_pr_curve(px, py, ap, save_dir='pr_curve.png', names=()):
     ax.set_ylim(0, 1)
     plt.legend(bbox_to_anchor=(1.04, 1), loc="upper left")
     fig.savefig(Path(save_dir), dpi=250)
+    plt.close()
 
 
 def plot_mc_curve(px, py, save_dir='mc_curve.png', names=(), xlabel='Confidence', ylabel='Metric'):
@@ -328,3 +330,4 @@ def plot_mc_curve(px, py, save_dir='mc_curve.png', names=(), xlabel='Confidence'
     ax.set_ylim(0, 1)
     plt.legend(bbox_to_anchor=(1.04, 1), loc="upper left")
     fig.savefig(Path(save_dir), dpi=250)
+    plt.close()