[datasets] Add IIIT HWS dataset

[felixT2K]

This PR:

• adds logic to load iiit hws dataset
• add corresponding tests
• add to documentation

requested by: @tobiascornille in #1049

dataset: Link

from doctr.datasets import IIITHWS
train_set = IIITHWS(img_folder="/path/to/iiit-hws/Images_90K_Normalized",
                   label_path="/path/to/IIIT-HWS-90K.txt",
                   train=True)
img, target = train_set[0]
test_set = IIITHWS(img_folder="/path/to/iiit-hws/Images_90K_Normalized",
                   label_path="/path/to/IIIT-HWS-90K.txt")
                   train=False)
img, target = test_set[0]

[codecov]

Codecov Report

Merging #1199 (986a1c8) into main (cc0ceee) will increase coverage by 0.03%.
The diff coverage is 96.00%.

@@            Coverage Diff             @@
##             main    #1199      +/-   ##
==========================================
+ Coverage   94.98%   95.01%   +0.03%     
==========================================
  Files         149      150       +1     
  Lines        6417     6442      +25     
==========================================
+ Hits         6095     6121      +26     
+ Misses        322      321       -1     

Flag	Coverage Δ
unittests	95.01% <96.00%> (+0.03%)	⬆️

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

Impacted Files	Coverage Δ
doctr/datasets/iiithws.py	95.83% <95.83%> (ø)
doctr/datasets/__init__.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

[odulcy-mindee]

Thanks @felixdittrich92 ! 👏