[BUG] Usage for filtered indices needs to be updated

[Asha Rani]

Expected Behavior

Tell us what should happen

Actual Behavior

build/apps/build_memory_index --data_type float --dist_fn l2 --data_path siftsmall/siftsmall_base.bin --index_path_prefix siftsmall/siftsmall_R32_L50_filtered_index -R 32 --FilteredLbuild 50 --alpha 1.2 --label_file ./rand_labels_50_10K.txt --universal_label 0

above commands gives me ,

--label_file option is not available

2. Even this : build/apps/utils/compute_groundtruth --data_type float --dist_fn l2 --base_file siftsmall/siftsmall_base.bin --query_file siftsmall/siftsmall_query.bin --gt_file siftsmall/siftsmall_gt_35.bin --K 100 --label_file ./rand_labels_50_10K.txt --filter_label 35 --universal_label 0
   is not correct compute_groundtruth is not used instead another file

## Dataset Description
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- Dimensions: 
- Number of Points:
- Data type: 

## Error 

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## Your Environment
* Operating system: Ubuntu
* DiskANN version (or commit built from)

## Additional Details
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