add unittest for fpn

tests/test_fpn.py

+import pytest
+
+
+def test_secfpn():
+    neck_cfg = dict(
+        type='SECONDFPN',
+        in_channels=[2, 3],
+        upsample_strides=[1, 2],
+        out_channels=[4, 6],
+    )
+    from mmdet.models.builders import build_neck
+    neck = build_neck(neck_cfg)
+    assert neck.deblocks[0][0].in_channels == 2
+    assert neck.deblocks[1][0].in_channels == 3
+    assert neck.deblocks[0][0].out_channels == 4
+    assert neck.deblocks[1][0].out_channels == 6
+    assert neck.deblocks[0][0].stride == (1, 1)
+    assert neck.deblocks[1][0].stride == (2, 2)
+    assert neck is not None
+
+    neck_cfg = dict(
+        type='SECONDFPN',
+        in_channels=[2, 2],
+        upsample_strides=[1, 2, 4],
+        out_channels=[2, 2],
+    )
+    with pytest.raises(AssertionError):
+        build_neck(neck_cfg)
+
+    neck_cfg = dict(
+        type='SECONDFPN',
+        in_channels=[2, 2, 4],
+        upsample_strides=[1, 2, 4],
+        out_channels=[2, 2],
+    )
+    with pytest.raises(AssertionError):
+        build_neck(neck_cfg)