Added optional input "model_dir" to class SpladeEncoder

[Grecil]

Problem

Everytime an object of this class is initialise, the model is downloaded again. This wastes time, memory and network resources

Solution

Added optional input "model_dir" to specify the directory model should be downloaded in and loaded from.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update
• Infrastructure change (CI configs, etc)
• Non-code change (docs, etc)
• None of the above: (explain here)

Test Plan

Describe specific steps for validating this change.

[Grecil]

Takes only 0.8 seconds to load now. Earlier it used to take atleast 100 seconds.