[Improvement] add option to keep tail frames

[dreamerlin]

This PR enables class SampleFrames with ability to sample tail frames.

Test Codes

from mmaction.datasets.pipelines import SampleFrames
from matplotlib import pyplot as plt
import numpy as np

f = dict(total_frames=1000, start_index=0)
s = SampleFrames(clip_len=1, num_clips=128, keep_tail_frames=True)

# Sample 1000 times
a = []
for i in range(1000):
    a.append(s(f)['frame_inds'])
a = np.stack(a)

# Count occurance of each index
u, counts = np.unique(a, return_counts=True)
b = []
for i in range(1000):
    try:
        b.append(counts[u.tolist().index(i)])
    except ValueError:
        b.append(0)

# Plot
plt.plot(b)
plt.show()

Results

[codecov]

Codecov Report

Merging #1050 (2cd57a1) into master (76db181) will increase coverage by 0.02%.
The diff coverage is 88.88%.

❗ Current head 2cd57a1 differs from pull request most recent head 195aad9. Consider uploading reports for the commit 195aad9 to get more accurate results

@@            Coverage Diff             @@
##           master    #1050      +/-   ##
==========================================
+ Coverage   83.12%   83.14%   +0.02%     
==========================================
  Files         135      135              
  Lines       10362    10369       +7     
  Branches     1775     1777       +2     
==========================================
+ Hits         8613     8621       +8     
+ Misses       1317     1313       -4     
- Partials      432      435       +3     

Flag	Coverage Δ
unittests	83.14% <88.88%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mmaction/datasets/pipelines/loading.py	88.95% <88.88%> (-0.16%)	⬇️
mmaction/datasets/pipelines/augmentations.py	87.63% <0.00%> (+0.32%)	⬆️

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